Don't remove any leaves. Later on, when the lowest leaves are looking a bit bedraggled and yellow, you can snap them off at the base. You should train the plants up string or canes with just one main stem, so remove sideshoots regularly. These don't look like leaves. They are leafy shoots that grow in the leaf axles (the points at which the leaf stalks meet the main stem). Just snap them off with your finger and thumb for a clean break. Pruning with secateurs is a bad idea as the cut stems will allow infection in.   If you don't remove sideshoots you'll end up with a jungle and a lot of small, unripe tomatoes. If you're growing outdoors in a cool climate (e.g. London), remove the growing tip once you see little tomatoes on the fourth truss. In a greenhouse you can let them grow to any size that's compatible with the space you have before removing the growing tip. Here in Cornwall I can still pick ripe tomatoes in my unheated greenhouse into late October.