Ive been training for only about 6months and im having a world of trouble against bigger guys, Im small 5'8" about 155 pounds and I can't seem to create any kind of offense against big guys. I normally endup on the bottom defending, what do I do to deal with the bigger guys? 
Arm drags
Stephan kesting has a great video set out called how to defeat the bigger stronger opponent or something like that. Some real good stuff on there. 
Awesome, thanks guys
You have to keep moving. I'm about 160 lbs. and train with a lot of guys bigger than me. You need to attack so they defend rather than putting their weight on you. I have a clip on my web site training with one of my training partners who is a good purple belt and 100 lbs. heavier, hopefully the clip will give you some ideas (the 2 clips - Gi and No-Gi are about 1/2 way down the Home Page under what does BJJ Look Like.
Karel (Silver Fox) Pravec
www.SilverFoxBJJ.com
You've been training for six months, there is your answer. And keep distance.
Arm drags are good.
Omoplatas, deep half, top game.
Sometimes you have to get way under them to sweep, but never let anyone ,especially big guys, cross face you. You will die.
I like knee on belly better than side mount for bigger guys.
You're supposed to be having trouble with bigger guys.
Nikem - You've been training for six months, there is your answer. And keep distance.
I think this is what you need to remember.
if you can't keep them from getting to posisions like side control or mount, there is usually a point where you know its going to happen, don't wait for them to get the position , then think of your escape. You know its coming, prepare before they can establish it. if you are 155lbs, keep moving. If they establish a dominate position and 200 pounds, you will have a hell of a time. You know its about to happen, don't wait and lay there and die.
I'm about 135. Just remember, you have to be the nail before you can be the hammer.
Focus on small steps to start. At six months of training, you're probably going to get passed, so focus on stopping his arm from going around your head as step one. That will stop the crossface and the killer shoulder pressure. When you are consistently getting that, you can work on step two (half recovery, under hook, escape, whatever).
Brick by brick.
Alot of great stuff, thanks guys
shoudawhouda -Stephan kesting has a great video set out called how to defeat the bigger stronger opponent or something like that. Some real good stuff on there.
+ 1 for Kesting's DVD on how to defeat the bigger, stronger opponent
^ +1 Kesting and brandon mullins video are good, arm drags, deep half and grip strength have all helped me (I'm 145 lbs)
Since you have 6 months training I'll let you in on a little secret.
It is hard to wrestle someone a lot bigger than you. This is why they have weight classes.
Welcome to Jiu Jitsu. :-)
From guard - feet on hips. From top don't get to tight because it's easier to get reversed or swept. But if you're looking for sport bjj advice I'd say duck the really big guys and stick to your weight class. I'd rather roll with a similar sized upper belt than a 220lb blue. Less risk of injury and you'll learn more cool sporty stuff.
Braxmax - From guard - feet on hips. From top don't get to tight because it's easier to get reversed or swept. But if you're looking for sport bjj advice I'd say duck the really big guys and stick to your weight class. I'd rather roll with a similar sized upper belt than a 220lb blue. Less risk of injury and you'll learn more cool sporty stuff.
Why feet on the hips? What is the advantage?
Take the back. That is the position where most of the weight and strength advantage is negated.