If you are looking to cover up or disguise a tattoo, the best thing to do would be to find a high-coverage concealer. A couple of brands which offer a suitable product would be Cover FX and MAC.
Concealers I recommend using
Cover FX cosmetics are widely used to cover scarring and severe blemishes on the skin, and are just as useful for hiding tattoos. The products also contain natural minerals and are actually beneficial for the skin!
MAC is a very popular cosmetic brand used all over: from high fashion photography shoots and the catwalk, through to the high street. They offer a very wide range of foundations and concealers so it would be very easy to find one to suit your needs.The celebrity tattoo artist Kat Von D has her own make up range which also contains a tattoo-cover up concealer, so I guess it'd make sense to check that out too.
When applying any concealer, I'd recommend using a primer underneath because it creates an even finish before you begin applying concealer.
It'll also help prolong the wear of the concealer.