I am currently slightly confused about the exact usage of the word "watashi". I know that the word means I, but it's usage seems unclear to me. Some sources say that it is simply for polite use, while other sources state that it is only for feminine use.
Can someone clarify this pronouns usage for me?
It's for polite use by both genders, and the most generally used first-person pronoun. While it used to be more for women, this is no longer the case.
It is true that women tend to use わたし (watashi) more than other pronouns, but it is not a feminine pronoun†, and it is frequently used by men. It is more polite than others and also used more generally. Typically, it is thought of as the first-person pronoun "safest" to use as a learner.
A useful quote from the Japanese Wikipedia article (linked below):
現在では男女ともに使用する。
Nowadays men and women both use it.
More gender-locked (but not exclusively so) personal pronouns would be ぼく (boku) and おれ (ore) for men, and あたし (atashi) for women. I would caution against using these, however, until you are more comfortable with the language, since they are a bit less flexible with the situations in which they can be used.
The answer given by that Yahoo! Answers post has four downvotes for a reason—its writer is a junior Japanese learner and does not seem to clearly know the difference. There's honestly lots of resources out there that talk about first-person pronouns in Japanese; avoid Yahoo! Answers whenever possible.
Here are a few resources on personal pronouns that are pretty accurate:
† It can be perceived as feminine if used in casual contexts.
Just a note: When saying something like "thanks in advance", it's better to say よろしくおねがいします (yoroshiku onegaishimasu) than ありがとう.
