As stated before, grip strength will come with time. However if you think it is a true weakness and is holding you back you might want to put some extra work into it. Grip strength is absolutly key when you start getting lots of reps of pullups, toes to bars, deads ect.
One thing I'll do from time to time is crab grip some plates and do a farmers walk. What this means is you hold the plates from the top with your thumb on one side and all four fingers on the other, not through the hole in the plate. Do a few sets where you carry them down and back your gym. Use whatever weights you can, 10s 25s 35s or 45s!