I second Miss M's advice. Your situation *sounds* incredibly simple. The only wrench in the works would be if you have some impressive interest-earning investments squirreled away that you did not mention.
Unfortunately, you can not write off "street clothing" used for work. No matter how dirty it gets! Trust me, I know as I am a bodyclipper so I'd love nothing more than to write off my destroyed clothing. However, there are some loop holes. If you are self employed, you can write off anything that you have embroidered with your "company" logo. (You need not be an actual corporation, you can just be "Sarah's Horse Grooming".) This puts your clothing into the realm of "promotional" material, thus making it deductible. This may not apply in your case, if you only had one employer, but it is something to keep in mind for the future. I embroider all of my shirts, ball caps, and the one winter jacket I use for clipping. At $10/a pop for embroidery, it allows me to deduct the entire cost of the item, logo and shirt price.
The other exception is any clothing that is considered protective, eg: work boots with steel toes. Standard paddock boots are deductible, even without steel toes as they are considered protective. Sneakers, in any form
, are not. The goggles I use for clipping are deductible, as are the earplugs I wear. If you lunge a horse as part of your duties, gloves would be deductible as well.
To answer your original question, if I were forced to ballpark what you will end up owing with no additional information provided to me, I'd say you'll probably end up right around the $400 mark. This would be self-employment tax, not income tax. Your income tax should be zero. A clever accountant will be able to get your SE tax down to almost zero too. So, it might be worth it in your case to have your return professionally prepared this first year, then if your tax situation remains unchanged, you can just subsequently copy what he/she did in future years' filings.
The other thing I need to comment on in your situation is if you truly had only one employer all year, then technically it is wrong for you to file self-employed. If the IRS knew the truth of the situation, they would flag your return. However, if your employer didn't file a W-4 on you, you really can't file as a statutory employee either. For the future, it would be better if you could get your employer to put you on the books. You can claim "0" deductions on your W-2 and there would be no withholding from your paycheck and no complicated paperwork for your employer. It's just the one time original set up of the W-4 and then the yearly submission of the W-2. That would make your tax return super crazy simple to do every year.
I worked as a tax preparer for H&R Block for three years, and I can confirm that yes, their fees are cheap, however, employees secretly refer to the company as Hit & Run Block. They will try to sell you extra forms (they charge by the form) like itemizing deductions when you really don't need to, so be alert and aware if you choose to go that route. Also, whatever you do, do NOT let them talk you into the "refund anticipation loan". It is a gigantic rip-off, they used to call it their "Rapid Refund" service and the government stepped in and took them to court to force them to stop calling it that. It's really just usury, pure and simple. Don't fall for it!
My father has prepared income taxes in South Florida for 30 years, he'd be happy to help you out and I guarantee he will charge you less than H&R and with out any superfluous bull. Feel free to PM me if you decide to do the form yourself, and I'll give you whatever free advice I can, or I'll send you to my father if I can't help. Good luck.