You'll be among the youngest, but you won't the only 18 year old there.
Be fit. The number of OCs that aren't astounds me. Nobody under the age of 25 should be running 2.4k slower than a 48 year old. I find the number that don't have the upper body strength to lift themselves out of a pool just as astonishing. You don't need to be superhuman, but arriving not able to pass the RNFT or Swim Test says a lot about the individual - none of it good.
Run and do some body-weight training. Arriving prepared physically will pay off in spades.
Get on top of your kit certainly (ironing and folding), but more importantly get on top of your time management and ability to prioritise. Folding and ironing kit isn't difficult at all, but to get it perfect takes a little time; spend that time loafing, scranning shit from the shop, or on your phone and you won't have done it perfectly. Too many people can't manage their time. Also, manage your time properly and you'll have the ability to stop and think, which is when you'll pick up the little things that need doing that can be the difference between being placed on Remedial Training or not. Good time management will reap rewards in every other aspect of training too.
Clutch in. Listen to what the training staff are telling you - and then do what they tell you. Don't think you know better. For example; the amount of OCs who on Dartmoor don't listen to the instructor about wet & dry routines, who think they'll be alright - who wants to put on wet kit? Those are the same OCs that are later going down because they've got no dry kit to change into - but why listen to the experienced Royal Marine Sergeant, it's not as if he's got any experience of staying alive and operating in the field is it?
Enjoy it. Some bits are more fun than others, but maintaining a positive outlook and mentality will make it easier. Engage fully with everything you're doing and be enthusiastic. Accept when you f**k up, accept that it was your fault, and learn from that so that you don't make the same mistake again, rather than wallowing in self-pity.