If the flapper( the rubber thing that comes up when you flush and slowly floats down during the flush) stops leaking when you push on the top of it, it is usually fixed with just a flapper replacement. Home Depot or Lowes has just the flapper for about $5ish. If you chose to only replace the flapper, turn off the water to the toilet, remove the old one,check the plastic lip/seat that it is supposed to seal to(sometimes the old flapper “melts” onto the seat), install the new one and adjust the chain to the right length. Flapper replacement is pretty staightforward, the package comes with instructions.
The plastic lip/seat replacment is a little more involved. It involves removing the tank and unscrewing a large nut that is on the bottom of the tank and installing a whole new assembly, including the 1” overflow tube the flapper attaches to. HD and Lowes has these kits also,$20ish like was said earlier. If you want to change out all the guts of your toilet with these kits, it will take an inexperienced person 1 to 2+ hours, if you follow the directions carefully. I have had good luck with the Fluidmaster brand, Model 400A valve kit
If just replacing the flapper doesn’t fix the problem and you think your skills are suspect, a competent plumber shoud be able to give a phone quote. It should take a good plumber less than an hour to install the complete rebuild kit(flapper, valve, handle,flapper seat, tank to bowl bolts). Try the flapper replacement yourself first. IF that’s all it needs, they will still charge a trip fee,usually, for a $5 part.