Hunt down and buy a copy of 'Africa's Most Dangerous' by Kevin Robertson. Unfortunately, it is out of print at the moment, so it will cost. This book is the bible of buffalo hunting. His other books, recommended in this thread, are also excellent.
Otherwise: if broadside, come up the front leg 1/3 of the way into the body. That shot will smash the shoulder, at least. If you want to be tricky, come up and then move back 6-8" in order to preserve the venison: BUT (1) the animal will take an instant dislike to you, and has a heart-stopping 2 or 3 minutes left to take his vengeance; and, (2) the venison is not worth eating anyway.
If face-on, or squared off to the left or right, aim equidistant between both front legs and 1/3 of the way up the body.
Check the diagrams to see how low the heart is against the sternum.
The alternative is to use monolithic bullets and a bolt-action, and brain it.
If arse-on, aim at the base of the spine, just above the tail.
But this is the theory. As Doctor Robertson says, once you squeeze that trigger the only certainty is that one of you is going to end up dead. The first shot is the aimed one: everything after that requires snap-shooting with a familiar rifle.
IMHO, buffalo (and kudu) is something you work up to. I should recommend that you take two or three safaris to get yourself familiarised with the set-up. The wildebeest is ofter referred to as 'the poor man's buffalo', and is a challenging game species (as well as being better eating): start with them.
But what do I know? I have shot precisely one buffalo, with a .470 double. That led us a merry dance, with a P-turn attempted ambush and a final charge: very, very, exciting: but it required a lot of mutual trust between the trackers, PH, hangers-on, and myself. That was, I think, my fourth safari.