You need to be an amazing artist first of all and tattooing on skin is so much different then drawing on paper try practicing on leather first
First of all you need a steady hand. A shaky hand will lead to jagged lines. You will also need a tattoo gun and some ink, and whatever size needles you think you would like to use. This may be bought on E-Bay for $150 dollars. If you are trying to become a professional, the best way to start is to either practice on your friends, or whoever you can get to let you practice on them, and then find an apprenticeship at a shop. If its something you just want to do for fun for friends and family you really don't HAVE to have a license, but I recommend it. I also recommend NOT reusing needles, they don't cost that much, and if you send the needle with your "customer" there is no chance that they can get any type of diseases from "dirty needles" since there will be none in the shop, room, where ever it is you are tattooing out of. Hope this helps, I know its not everything one needs to know to start tattooing, but its a start, any questions let me know!
You need to know how to scech extra good and need to have experience