there are plenty of tour operators like Somak and Sunvil that book safaris. The important thing is for you to work out just what you want for yourself. What sort of price you want to pay, what animals you'd most like to see, what sort of accommodation (luxury/roughing it, mobile or fixed tents, big hotels or small lodges etc), what sort of sightseeing (walking/driven), what time of year (because animals aren't necessarily in the same place all the time) etc. Tanzania has plenty of open grassland - good for game-spotting - but much less tourism infrastructure than Kenya, for instance. In general, brochures try to explain the different things you choose from, but don't be afraid to ask travel company staff. Often smaller operators are more specialist and know the places better. You could look at
www.aito.co.uk for some of these.