I don't know what the rules are for this match, but here are some basic things I've learned over time. I think they're good things to have in mind for anyone inexperienced in this sort of thing.
A simple take down: all you have you have to do is grab onto their pant legs, one hand on each pant leg. Don't even grab the leg, just the pants. Or if they're wearing shorts, the bottom of the shorts, whatever. Now, keep the legs still; pull them towards you, whatever you gotta do. Then basically just run forward with your head down a bit, get your shoulder into them. Since you're already bigger/stronger than the guy he should fall down if he can't get out of it fast enough, just make sure you do it fast and hard. Then capitolize. Never let a downed opponent just sit there, GET onto him, just make sure you have a plan. Going for the mount is generally a good idea.
If he's trying to take YOU down, maintain an athletic position. Do not stand up straight, or jump, or anything silly like that. If he makes a rush at your legs kick them backwards behind you and grab his upper body/head, use it as a support for a moment just to keep your legs away. Guillotine him, it's especially easy to do against people rushing towards your legs.
Watch your neck and don't get guillotined yourself. In a situation where you think your neck is exposed, always be ready to move your hand to your neck, BEFORE his hand gets around it. If your hand is even just next to your neck he's going to have a hard time grabbing you there.
On the ground things can get real confusing real fast. A few important things:
DO NOT GET MOUNTED. No matter what do not let him straddle your waist; accomplish this by keeping your legs high when you need to, and look for when he's trying to get a mount. If you fall backwards, fall with your legs up: try and get them around his waist as he falls on you. Getting taken down is far from defeat: you can quickly turn things in your favor.
If anything, on the ground, if you aren't trying to go for a leg lock or an arm bar or what have you, you should be trying to mount him, or get behind him. If you can get behind him, wrap your legs around his midsection and lock your arms around his neck or give him a full nelson or something. He's not getting out.
Be limber, be like a spider; crawl all over him, get into position. Lean on his back and use him as a pivot to turn around. Get behind him. You are mobile; don't forget it.
Don't panic. Don't let your adrenaline get the best of you. If you give into instinct chances are you won't have the sense of mind to do anything useful other than waste MASSIVE amounts of energy. Always try to be aware of what you're doing, what he's doing, potential openings and etc. Keep a clear head.
On that note DON'T WASTE YOUR ENERGY. If it looks like you can't bring him down a certain way, don't keep trying over and over. Don't overexhert yourself attempting to do things like heave his entire bodyweight in a particular direction, lift him above your head, or stand up with him on your back or something. You can throw him around all day, but it won't end the match; the match ends on the ground. Don't let him fool you into wasting energy. No matter how strong you are, if you're winded and burnt out you're going to lose.
And be careful. You guys are friends.
Do your friends wrestle for school, btw? Are they going to use standard wrestling rules?
