Mainly with the items, you want health, ap, and cooldown reduction. I go with magic penetration to make teemos mushrooms do that much more damage, as well as the increase in damage for teemo all around. With this build you can change the items to get more AP or more health, whichever to fit your playing style but you want to keep the cooldown reductions to keep your mushrooms coming up.

As with almost any champion in League of Legends, if your team sucks your not gonna be able to do much with Teemo. Other than that there's really only one con about my build here, no life steal or spell vamp(that I can think of). There is one good thing about this build, It makes you not squishy so that you can have pretty much no fear of running around in the jungle dropping shrooms like a mad man/woman.

I never really jungle with this build, could be done I suppose. Creeping is really easy at around 6/7. Be sure you place a mushroom every chance you get while creeping, and replace shrooms that have been hit by the enemy team as soon as possible. Try to keep a mushroom near all the buffs lizard/dragon/golem/baron, and replace those as needed.

Team work is extremely important with any match on LOL. When I go to help out team mates who are on the verge of death, I try to use blinding dart then one regular attack with a mushroom on their retreat path(guess the path they will take to leave (will help you get kills and avoid ganks by enemy team). Since you have alot of cooldowns with this build your main way of helping your team is by making a mushroom mine field of the entire map(if possible...). One good way of helping the team is by placing 3 or 4 mushrooms around your tower wherever it may be and running up to their lane to gank.