Not a Marine...
I served The Queen in Infantry and Special Forces Group, so these bits of advice will help you out.
Rest. When you are told to rest, rest. The DI will tell you when you should maintain your kit and when to run around playing "Silly Bugger".
Care for your feet. As a Marine you will do a lot of walking, running, marching, crawling, more running and jumping. Keep your feet dry as much as you can, bring spare socks in unlubricated condoms (Trust me on this).
Watch out for "buddy". You are going to be part of a team, watch your fellow Marine's back, 'cause if you don't and he dies, who's gonna watch yours when your number's up? I am serious.
While you don't want to be a "Super Star" you want to do your best. Don't worry, for the first six weeks it won't be good enough, that's the way of IMT. Any body who doesn't do it like they mean it shouldn't f*cking be there.
Watch, listen and learn. Watch what the DIs do in the field, listen to what they say and learn where they hide the booze. Only half joking here. These Marines have been in the service long enough to be trusted with training you lot, so they are switched on.
Good Luck.
When you are ready for the sharp end, drop me a line. I'll give you some pointers on how to make it through a desert war.