Don't like assuming, so here are a couple suggestions...
If your handle, chain or linkage are free and clear, not causing problem with the flapper then it's likely the flapper needs replacing. I have seen times when handle is defective and the chain ended up between the flapper and flush valve, so if handle and chain aren't problem likely the flapper
Question: Are you using chlorine drop-ins designed to keep tank, bowl clean? If so, remove them and DON'T USE THEM EVER AGAIN. Chlorine eats rubber. I've had to change many flappers on brand new homes because of these things. Good for me but bad for my customers. When you touch the flapper if it is real slimy or the rubber stains your fingers, blame chlorine,either over-chlorinated water or drop ins.
One other thing. The thin rubber, plastic tube coming off of your ballcock, and goes to the overflow tube, is it shoved down into the overflow tube or is it clipped just above the top of tube? It should not extend below the water line or it will siphon water and cause ballcock to turn on and off periodically.
You can also turn the supply valve off after tank is full then use a pencil, mark water level, don't flush toilet, come back in 15 minutes and see if water level dropped...if it did, replace the Flapper.
If you are not sure how to remove and replace the flapper be sure to google the toilet and find out proper removal....most are currently mmon sense but these days there are so many manufacturers that it will save headaches to do a quick check.
Hope this helps. Good luck.