Everyone here is right. If you have rear suspension, have someone sit on your ped and load and unload the rear suspension until you figure out where the chain is tightest, usually when the drive sprocket, swing arm spindle and rear axle line up. You can also load the suspension with a tie down strap from the swing arm to the frame or seat.
After adjusting the cahin, rotate the rear wheel and check for the least amount of chain slack. If it varies a lot, you most likely need a new chain and/or sprockets. Then, with the chain in the tightest position, load the suspension again and make sure you have a little play left.
Too tight a chain will tear up the chain, sprockets and bearings at both ends before you even notice a loss of power.