You can't. You'll only be able to choose 50 perks out of a total of about 280.
i think thats awesome, ive always tried to stick to the main path or w/e for the character type im playing but find it almost difficult not to increase everything to the max in the older games. with the new system only allowing you 50 out of ~280 or so then that seems like it would create a huge amount of different unique characters whether they want to be extremely skilled in one aspect or only mediocre in all of them, i think its a great addition and idea.
The one question i have about the whole perk tree system is if we will get to see the entire line of perks for that skill or attribute before we start choosing, like if theres one really gangster awesome one you want to shoot for or if you really just have no clue going in right from the start and you just stick to what you like and new perks just come up along the way and you discover new ones.
edit: without knowing, im sort of undecided on what id rather have....i like the idea of only being able to see the perks as you level, giving it a sort of unknown exciting reward aspect, but on the otherhand it could also be good to have be able to see the entire thing and plan out which direction you will take