First try tighting up the bolts under the tank. Just tighten each one up equal
turns, do not over tighten.
You may have to shut off the supply valve, flush the water and hold the phillips head on the bolt and tighten the nut at the same time.
If those tank bolts were not tight enough, the tank would leak all the time?
Based on what you said, it might be that the tank-to-bowl gasket might be no good.
Replacing it is not difficult. Beforehand, I would go to a plumbing supply store and give him/her the make and model of your toilet and let them give you the correct replacement part. Or go to a box store and get some generic ones for trial-and-error fitting.
I would also suggest:
1. replacing the grommet/gasket for those tank bolts at the same time as you would need to remove those bolts anyway for this project.
2. replacing the water supply hose (get a braided one that is long enough to loop)
3. buy plumber's silicone grease and apply it on your new tank-to-bowel gasket to minimize drying/cracking