I love dragons but I don't want to be a full dragon or a full human all the time so I want to play as a 1/2 dragon, 1/2 human but to clarify what I mean by 1/2 Dragon is human form but just add Dragon tail, claws, horns, and eyes, and maybe wings. Also my character has the ability to either be full human or full dragon but can't sustain it for prolonged periods of time without passing out from the mentula strain.
PS the PC I'm making is not an adult but a teenager in the 13-16 range.
A "half-dragon" in 5e might be:
Most often, sorcerers with this origin trace their descent back to a mighty sorcerer of ancient times who made a bargain with a dragon or who might even have claimed a dragon parent.
Born of dragons, as their name proclaims, the dragonborn walk proudly through a world that greets them with fearful incomprehension.
Neither is close to a true dragon in terms of power, however, both have a distinct draconic flair.
Also, Dragonlance setting has its Draconians. I don't know what setting do you play.
A worthy addition from the comments:
A Monk archetype from Fizban's Treasury of Dragons:
The fundamental teaching of this tradition holds that by emulating dragons, a monk becomes a more integrated part of the world and its magic. By altering their spirit to resonate with draconic might, monks who follow this tradition augment their prowess in battle, bolster their allies, and can even soar through the air on draconic wings.
You could even combine all three (thanks to @Kirt for the example):
have a PC that is a dragonborn monk (way of the ascended dragon) / draconic bloodline sorcerer (currently level 1/2). The idea is that he is becoming more dragonlike as he increases in power
Regardless, there is no legit way to turn a low-level PC into a dragon. Adult dragons are very strong in D&D (5e in particular), so making dragon a PC almost certainly breaks the game until very high (17-18) levels. However, you can become more and more dragon-like.
There are no official player options that do this unless you are playing in a high level game. A 17th level spellcaster with access to true polymorph or shapechange can use one of these spells to change into an adult dragon.
Others have already addressed this, but there is no way (within the rules) to make a level 1 character be able to transform into a dragon.
The only way I can see you getting the power you want is through some sort of homebrew, one that you can search for or one you can create with your DM. A power of this magnitude is not something that an early game character should have. Here's an example of how I would handle a request like this.
Somehow, for some reason, before the campaign begins, your character his gotten on some powerful spell-caster's bad side and you've gotten cursed, transformed into a young human and severely weakened. Now with the strength of a level 1 PC, you begin your search for someone that can undo this curse. This can be a major plot hook for your group's campaign, or a personal side quest that your party can assist you with.
As for stats, you've been weakened to a standard human, for now. As this specific plot line progresses during the campaign and you gain levels, I as a DM would unlock different draconic powers in addition to your chosen class features. Here are some examples of what I'd unlock early on:
This is just what I could think of off the top of my head, it would be up to you and your DM to come up with the abilities you want and how you would gain access to them. Getting the features of such a powerful creature is going to take some work to make sure you don't completely over shadow your teammates in your party.
First - I'd suggest talking with your DM about it. My experience suggests that attempting to bruteforce character, who is not to DM's liking into game by hitting DM with rules till they'd give into it - is a bad way to start a game.
Second - you can ask GM to 'reskin' one of existing races, saying that you'd take their statistics, but will look like a dragon. I'd suggest trying Aarakocra from Mordekainen Presents: Monsters of Multiverse - they have both wings and flying speed, plus unarmed attack with their talons, which makes them perfect candidates to be reskinned into humanoid dragon.
Third - there is an option 'custom lineage' from Tasha's Cauldron of Everything which gives you full control over your character's origin and looks - but in that case those would only cosmetic options - no real flight or anything.
Only one race (afaik) can shapeshift as a racial ability - those are Changelings, but in their case change is only cosmetic, they don't get any special abilities of those they change into.
Advice: From what you wrote in this question, in the comments, in chat and in another question, it seems to me that you are a newbie in 5th edition of DnD: I kindly suggest to read the Player's handbook, in order to have a general sight about the rules. Moreover, I suggest then to bring this character idea to your DM and work it with them, in order to achieve a playable and balanced character.
Your DM may adapt the Half Dragon template in the Monster Manual: from the description of such creature we can read:
When a dragon in polymorphed form mates with another creature, the union sometimes produces half-dragon offspring. A creature might also transform into a half-dragon as a result of a mad wizard’s spell or a ritual bath in dragon’s blood. In all these cases, the result is a creature that combines the essence of a dragon with the form of its original race. Regardless of their origins, all half-dragons have similar features and appearance, gaining special senses, resistance to destructive energy, and a breath weapon.
It gains more senses, resistance and the breath weapon related to the lineage of the dragon bloodline (chromatic or metallic). Using this template, the DM may add more details and/or features, as for example the usage of wings once a certain level is reached1, or looking more dragon-ish a limited number of times per day.
Nonetheless, the DM has to take into account that this template covers the creation of monsters/NPCs, that usually follow different rules with respect to player characters's one.
I would pay particular care in allowing becoming a true dragon: in the first tiers of play this could be really unbalanced, dragons are powerful creatures2, as Illustro notes in their answer.
There are some spells (True Polymorph, Draconic Transformation) that allow to become a full dragon, or even magic items (e.g. Potion of Dragon's Majesty, as stated in this answer to the other question of yours), but they are powerful and available at high levels, they are not suitable to low level characters, particularly for 1st level ones.
1 Check for comparison Drows or Tieflings, that gain the usage of a 2nd level spell once the 5th level is reached, regardless the class.
2 Well, we are playing Dungeons and Dragons...
If I wanted to focus on what you call the 'strain' of your character hiding/showing their features right from level 1, I would start with the Custom Lineage and take the Eldritch Adept feat for the Mask of Many Faces invocation and use the disguise self spell to constantly be shifting in appearance between my more human or draconic sides. Custom Lineage also lets "You determine your appearance" with your initial cosmetic features, so you can tailor your underlying 'default' half-dragon look.
If I wanted to focus more on practical dragon features like breath weapon or flight at character creation, I would start with the (Gem) Dragonborn (breath weapon, and flight from level 5) or Winged Tiefling (fire resistance and flight from 1st level).
For the feeling of occasionally losing control and becoming a massive dragon-like beast, I actually think playing a Moon Druid would be fun. Work with your DM to add dragon flavour to the wild shapes, like Deinonychus dinosaur at level 2 and the huge flying Quetzalcoatlus dinosaur by level 8. Cantrips like Primal Savagery (claws and teeth), Thunderclap (roar), and Create Bonfire (fire) all fit the build when not wildshaped. Normal stats for a druid would be Wisdom highest, Constitution next, then Dexterity.
To focus more on fire, go with the Draconic Sorcerer. Take the Fire Bolt cantrip for attacking and setting everything else on fire from level 1, the Dragon's Breath spell by level 3, and then cast gargantuan Fireballs from level 5. Stats for a Sorcerer are normally Charisma highest, then Constitution and Dexterity.
