I'm not an expert, I'm self-taught, but learned a lot in the past 20 years. Now I do quite well, and have built suspensions, modified and built custom frames, roll cages, trailer hitches, and a bit of body work. You CAN lean yourself, but it will take a lot of trial and error and patience. Get lots of scrap metal similar to what you will be working with and experiment.
For welders, there is a huge range of sizes and prices. I spent my first 5+ years using a small, 110v mig from Sears. It was a Sears name but made by Century. Its about the cheapest I would recommend for a usable welder. It was around $200 at the time.
For sheet metal you want gas. Shielded wire (flux-core) burns hotter and doesn't weld nearly as cleanly, so you want to avoid that for sheet metal. Use the smallest wire available, usually .023" and a low heat setting.
You can do a fair amount even with a little 110v mig, but the downside is that if you are doing heavier welding you will end up with a long duty cycle- you will weld a short distance and have to let it cool down for a while. That can end up damaging the welder after a while if you push it too hard. For a good 110v machine, look at some of the small Lincolns, like the sp100 or sp125. A little more money, but they will work well and last.
If you might end up welding more in the future...and trust me, once you have a welder you'll be welding everything...spend the money now for a slightly larger, 220v machine. For home welding projects I have the Lincoln SP175 Plus. That will weld everything from sheet metal to truck frames, but is still portable.
One piece of advice- use a GOOD power source. You can plug a small lincoln into a 15a circuit and do a little welding but it will be poor welds and the power will fluctuate. You also NEVER want to use an extension cord, unless its a large commercial one rated for the power. If you are in a bind, use some Romex and a couple plugs to make one- it will be much better than a light duty cord from WalMart. I shouldn't say this, but that's actually what I did for at home, since my only 220v outlet is in my basement.
Edit: Link to the one I use now. This is a good price, but it's probably more welder than you need to start out.
http://www.airgas.com/browse/productDetail.aspx?Category=231&product=LINK2300-1
And this is similar to my first welder. Boy have the prices gone up...for the price, I'd skip it and go name-brand now.
http://www.sears.com/shc/s/p_10153_12605_00920569000P?prdNo=5&blockNo=5&blockType=
Good luck.
Jim