There are several methods of covering up tattoos, but there is a common one. Buy several different shades of concealer, a concealer that is lighter than your skin and darker etc. buy a powder, use a brush to blend it. I personally have never done it myself, but I will link a few sites that may be able to help.
Other tips is too try different methods and brands of concealer and see which one works the best for you, on top of that like all make up you will probably have to go in every couple hours and reapply some concealer, its a routine thing to do. I wish you luck! If make-up sn't your style, there are Tatoo "Cover ups" they make.