I'm sorry for asking such a simple question, but I'm new to Blender, and even after much research I couldn't find a solution to this problem.
When I try to delete a specific unneeded object in object mode, all of the objects I entered get removed. How can I get rid of the unneeded object without getting rid of the vital ones?
In Object mode you can create several objects, and to delete one or several of them, just select and press X.
If you select one object and enter in Edit mode, you can create several distinct meshes within this object, "distinct" meaning not linked between them by any edges.
Example: in Object mode, create a cube. Then select it, enter in Edit mode, and press shift A to create another cube within the object. The object now consists on 2 cubes.
As explained by Ashutosh, as long as these meshes are distinct you can select one or several of them, hovering your mouse over their vertices, edges or faces and pressing L. Or click one of its vertices, edges or faces and clicking ctrl L.
Now you can delete what you've selected or make it a separate object pressing P (Separate) > Selection. If you select all the meshes of an object and press P (Separate) > By Loose Parts it will create as many objects as you have distinct meshes.
Did you add that new shape when you were in edit mode of another object? Adding a new shape while in edit mode, adds it to the object that has been previously selected when you entered edit mode. I'm not sure how to select whole objects while in edit mode but maybe this helps.
