Self assessment, first time help.

Evening everyone, newbie here.

I'm self employed (subcontractor) trainee and earn £15,000 a year but have been doing overtime. I'm going to be doing my self assessment (or giving it a try) this year but am confused.

The guys at work, from experience have said that because of the money I earn I should be better off than someone who earns more and because I'm only a trainee, why is this?

I get the 20% tax and have been working since July 2012, and from April to July 2012 I was receiving DLA.

I've kept receipts for fuel usage (but not noted mileage) And train fares.

If I find the online bit too confusing I'll probably pay for someone to do it, but awaiting my pass code for the online section.

Thanks in advance

(please move if wrong area)