Do you know how to climb a rope?  If not, you will have to learn in a high-stress environment while dog-tired, with DIs yelling at you the whole time at OCS.  Learning how to climb a rope before OCS will help you on the obstacle courses, and help mentally prepare you for the training.  You can see a Marine demonstrate great form here:

Rope Climbing Instructions

First of all, having a good pair of boots helps!

The rope itself is going to go between your legs, out around your knee/calf and back between the insteps of your boots.

With the rope like this, clamping your feet together acts like a brake and ideally, you should be able to support yourself in this position without using your hands and arms.

The basic idea of this drill is that you are going to ‘inchworm’ up the rope using the “brake and squat” technique. Climb up the rope by bending your legs, loosening the brake between your feet. Then once you straighten your legs, pull yourself up the rope about a foot or two and repeat until you reach the top.

Beginners should get “the hang” of this technique in no time.