After you hit lvl 7 - 8 it is time to start either pushing or ganking. If at all possible, ask your jungler for Blue buff. If you have any spare gold, pick up some wards.
As Elise, your job in team fights to neutralize their carry and clean up after your team has gone in. Due to the fact you have extremely high burst, you should be focusing a squishier guy like Ashe, Caitlyn, LeBlanc, and Veigar. Your Cocoon should be saved for when you see a carry or high priority target grossly out of position or exposed.
REMEMBER THAT POSITIONING IS KEY
For the first half of your combo, you must be near enough to the team fight to deal damage but far back enough to not be focused or exposed when doing so. The second half of your combo though forces you to go directly go into a fight, so you'll have to time your entry and use of Venomous Bite very carefully.
You may not always be able to pull off your full damage on only one carry, so don't be afraid to try and hurt a support or someone squisher at the front. By the later stages of the game, you should play extra cautious and always be on the lookout. Stay at the back of your team and focus the enemy carry, or the person you think deals the biggest amount of dmg.
If the enemies backline is too well protected, stay back and focus down whoever jumps in. Sometimes it's better to just focus down whoever is closest as a team than to just blindly jump onto the enemy backline, though if their carry shows up, by all means focus them instead.
DON'T CHASE! You'll cut off a huge amount of your teams overall damage, expose yourself, and more than likely lose the fight/get killed. Let them go, your team fight is more important than one potential kill.
Try not to fight when they have baron! The buff is really powerful and only unless you have to or are in a 5v3 situation, DO NOT ENGAGE OTHERWISE!
YOU ARE NOT THE TANK! Do not engage! Too many times I have seen people wandering in the front of their team, only to be jumped. A good rule of thumb is to stay a little behind your tank or support, depending on who they are. This way you won't get caught and can get right into the upcoming fight.