Fixing a toilet tank leak should be a fairly simple task, take the tank lid off and watch where the leak is coming from. Usually it is the bulb and that is a fairly easy task to replace that piece.
First find out where the tank is leaking. Drain the tank. There is a cement compound that you can fix the leak. When it is set you can fill it up with water and it should be okay. Otherwise by a new tank. There are ones that are water savers.
Most commonly it is caused by a damaged gasket. After you locate which one, shut down the water to the tank, remove damaged gasket and take it to your hardware store to ensure to get the correct replacement. Then, install the new one and turn the water back on.
When you buy a toilet it generally come in two parts, the tank and the bowl. If the tank is leaking it's usually where they are connected together. There is a gasket between the tank and the bowl. It's connected with two bolts to either side of gasket.