Matchmaking usually tries to set up an even game, it wants to encourage the most competitive game it can.
I didn't have a 360 for a few years (I had to pay rent and was broke) and had to resort playing at a friends while he was doing something else (like WOW) or played together as a guest. Since he was highly ranked and I was new to the game, I was getting my a** handed to me every time. Eventually playing against really good players over live, it builds your skills.
On other occasions I've gone over another friend's place and played halo with him as a guest and wondered how I was getting killing sprees so easily. Turns out his rank was a Sergeant while my previous friend was a Brigadier.
Tips I've learned.
-Learn the maps, learn the maps, learn the maps. - Each map has it's own strategy.
-Don't rely on power weapons, if you can't kill someone with the weapons you start out with, you're in for a rough time. The battle rifle is a great weapon to have at all times.
-Never just run and gun. Hid behind cover, crouch to hide on the radar.
-Practice with your grenades. A well placed grenade will either kill or take away the enemy's shield.
-Lone Wolves is a good mode to train in my opinion, it teaches you to rely only on yourself.
I noticed a huge difference in the way skilled players play vs new ones just by playing Halo on other people's xbox's.
Oh and don't let any a**holes get to you by talking sh!t, all these little punks get real brave behind a microphone.