So this is my problem, I play as a zerg and almost every time I meet terran player they block of the entrance to their base with different kind of buildings. Now I know using banelings to take them down is a well used tactic but they always have a bunch of siege tanks and bunkers behind them. So I try to go in with a mutalisk attack but the terran anti-air is just to strong. The terran player then comes out with a big air attack and the match is over. Please keep in mind that I am new to this game and has just started my placement matches.     

" So this is my problem, I play as a zerg and almost every time I meet terran player they block of the entrance to their base with different kind of buildings. Now I know using banelings to take them down is a well used tactic but they always have a bunch of siege tanks and bunkers behind them. So I try to go in with a mutalisk attack but the terran anti-air is just to strong. The terran player then comes out with a big air attack and the match is over. Please keep in mind that I am new to this game and has just started my placement matches.      "

In Zerg vs terran I normally go for a heavy Roach build. Roaches are pretty good against marauders and tanks especially if you can burrow and regenerate them. After the tanks are out of commission it's probably a smart idea to throw some zerlings with the speed upgrade into the mix, preferably to take out his economy. 

If you practice you should be able to get the banelings out before he has any siege tanks. If that fails, I would suggest you take advantage of his defensiveness; expand all over the map and just out-produce him. Harass him and exploit his immobility, nydus worms are great for that. Hope this helps. 

Most of the time if your very quick about it you can get your zerglings to his base before he has much of a defense blocking off the entrance.

" In Zerg vs terran I normally go for a heavy Roach build. Roaches are pretty good against marauders and tanks especially if you can burrow and regenerate them. After the tanks are out of commission it's probably a smart idea to throw some zerlings with the speed upgrade into the mix, preferably to take out his economy.  "

if he is turtling that much then expand all over the map with hatcheries and build multiple spires so that if he takes out one you can still put out mutas.

Zerglings with speed and banelings are a good way to counter early terran aggression.  If he's going Marine heavy, you can go roaches, but it's a gamble.  
 
Mid game is a bit harder.   Zerg has to work it to destroy the Bio-ball.  First of all, you need Infestors.  Fungal Growth > Bioballs.   Once they're trapped, you want to use Zerglings and later Ultras to shred them. 
 
Mutas are good harass units, and good if they're going ground heavy, but Marines can shred them.  Hydra are also good, but you don't see them quite as much Agilent Terran, because again, they aren't quite as good as Marines.

"\ Now I know using banelings to take them down is a well used tactic but they always have a bunch of siege tanks and bunkers behind them. .      "

thanks everyone, lots of great tips