How do I get the toilet to stop filling?

The tank on my toilet keeps filling. I replaced the ballcock but it still trickles. I have tried adjusting the float but I can't get the water level right and can't get the ballcock to stop trickling. I feel like I keep going back and forth. What is the trick to getting the right water level and the mechanism to completely shut off when the tank is at a proper level.

Your flapper valve is leaking. That is the rubber flap at the bottom of the tank that lifts when the handle is depressed.

Is the chain the right length? Are you getting a complete seal at the bottom of the tank? If it is seeping out there, it will continue to trickle in.

The first three responses I was able to read before answering here are good responses. I know from first hand experience throughout life. More importantly though, and maybe you already know this, you DEFINITELY NEED TO FIX IT because that slow leaking dramatically INCREASES YOUR WATER BILL, VERY MUCH, so, whatever you have to do, DO IT, or you're going to have a MUCH LARGER water bill than you're prepared for. I once lived across the street from an old man who lived in a small frame house with one bedroom and one bathroom, and he lived alone, and he came to me one day asking how his water bill could be so high. I think it was in the $90+ range or more. And this was in the late 1980's. Not being a plumber then (nor now) I went to the city hall and asked the water department, and they said he most likely had a commode that was never filling completely and therefore turning the water off. I raced back to his house and he let me look at his commode, and, sure enough, his commode was running water continuously in his tank just as you describe yours, and I fixed it with bending a rod or two, tightening the bulb on the end of one of those rods and adjusting something, and his water stopped. Next month, his water bill was in the $20+ range. So, get it fixed ASAP. Best wishes to you. God Bless you.

Stop trying to fix it yourself call the plumber

Replaced your flapper in the bottom of the tank. Make sure your changed from your handle to the flapper isn't to tight. If it keep running you should by a replacement valve the cost about six dollars

Bend the ballcock down, or adjust screw on mechanism

The problem is not - your toilet filling up.
Rather it is trickling down ( a leak ) .

Replace the flapper in the bottom of the tank.
they break down over time from water, and its made of rubber. When this flapper leaks , it allows water to trickle into the toilet, slowly draining it ...and then the ballcock ( float assembly ) tries to re-fill it again.

Since you didnt mention water on the floor -
I'll guess its not the tank gasket or bolt gaskets leaking.

After u replace the flapper.....
with the top still off the toilet - lightly hold up on the float assembly to insure it is off -
put your ear close to the tank and listen.
You shouldnt hear any water trickling sounds.