Okay, finally something I do well. I'm a custom painter. If you want to go cheap you can get a great look with rattle can paints. The only difference is the amount of paint a rattle can lays is much thinner than that out of a spray gun.
Depending on the state of your tank (I'll use tank as a reference, it can be any tin) you can just scuff, or you can take it to bare metal. A paint job is ONLY as good as the prep.
LACE PAINT
Sanding: Use an 80 grit sand paper (dry) and sand the tank until it is uniform and there are no low spots. (use a lower grit to get it to bare metal) Then use a progressively finer grit to make the tank smooth but with enough "bite" for your primer to adhere to. (If you have any dents that need fixed you will need to go to bare metal. I can explain body work if you need it.) when you get to 180 grit lightly spray the tank with your primer and re-sand to find any low spots, they will show as the areas that there is still primer. Once you have the tank surface level increase to a finer grit. You will want your sanded finish to be at a 600 wet/dry
Priming: There are many primers, a good one for a beginner would be a high build sand-able primer. The color doesn't matter so much, but I would stick with light colors for light base coats and dark for dark. Plug all holes and tape any O.D. threads. Hang your tank where it is at about chest level with a heavy duty wire or rope. Before priming or painting make sure your area is as free of dust and debris as possible.
Shake the can for the full two minutes. Hold it about 6 to 8 in. away and apply a LIGHT first coat and allow it to flash (tack up) for about ten mins. When using rattle can paints a lot of light coats are better than the flash, med, and wet coat when using a compressor and gun. apply another thicker coat and let flash, continue until you have an even, fairly thick layer of primer.