We were fortunate enough to have the funds to do one in September. Your 1st question should be what is your budget as Safari's are NOT generally cheap! Although you probably could organise it yourselves we chose to use a reputable firm (Kuoni - they were top drawer, Audley are another very good firm). A key part of the Safari are the guides that drive you round, research noted there are not so good ones, another reason for using a reputable firm. We did Kenya as a multi stop. 2 days Nairobi, 3 days in Samburu and 3 days in Masai Mara. Our guides were fantastic and really made the experience out of this world.
Top tips: you will be up very early every day (before 6) also there are night drives, take a good camera, definitely wear the right clothing!. The main advice we were given was you will need a holiday afterwards - yep we were knackered, we hopped over to Zanzibar for a rest (4 days beach holiday).