So i'm going to be getting a job, I need help with one thing, am I allowed to use my preferred name on a job application or legal name (also how do I make it so the managers or the people working there call me and know me by my preferred name?)

It's a legal requirement you put your legal name on your paperwork. However, I have seen it done this way:

James Alan ("goes by" Keith) Smith

You have to put the words "goes by" in quotes. You have to write it exactly like I have it with the parenthesis too.

But it's better in the end to do it like the other answers say. It's much more professional. Sometimes, the paper work will already have a "goes by" already in place to write that choice of "goes by name".

Good for you! I always wrote my legal name on the forms.

First Name:  HappyTwo  Last Name:  BeHereTwo

If a place for your preferred name isn't provided, you can write it parenthetically.

First Name:  HappyTwo (Happy)  Last Name:  BeHereTwo

or just verbally introduce yourself that way. 

Good luck!!!