Why is your toilet running? Do you have to "jiggle the handle" to get it to stop? I'll show you some of the most common reasons for this and quick and easy fixes. Reference the picture below as I explain everything.
1. The flapper may not be going all the way down to stop the water from draining from the tank. One reason for this is could be the chain, I've seen people not give the chain enough slack so the flapper may not be able it sit all the way down. Or the chain has too much slack and it will fall down under the flapper so the flapper will sit on the edge of the chain leaving a gap. Look at where the chain hooks to the handle arm, there's a clip where you can adjust the chain length.
2. Float is getting caught on something. Check the float and float arm and make sure they aren't rubbing against something or getting stuck by coming in contact with anything else. If one is, you can adjust it to where it has more room or adjust whatever it is getting caught on, sometimes you can slightly twist something out of the way at the base to turn it. Be careful by trying this and try not to break anything.
3. Fill tube has too much slack and is catching something. Check the fill valve and make sure it is not in the way of anything else.
4. Water is leaving the tank not through the flapper. Check underneath the tank where the tank attaches to the bowl. Is water coming out close to the tank bolts or where the fill valve and water line hookup is? If so, One of these nuts may be a little loose and letting water out there, simply tighten the nuts but not too tight because the porcelain is fragile and may crack.
These are some of the most common ways that I have seen that will cause toilets to run. Check these out and if they don't fix it, you can post a reply below and I will be happy to give other options and possibly guide you through it.