...a plan. You'll have to shut the water off to the tank, drain what's in there now (flush, then use a bucket below or sop it out with a big sponge). Whatever new rig you buy should have instructions, or find internet instructions like these. Maybe an hour's work, tops. Advice: do not overtighten connections--you can stress/crack stuff. Finger tight, then a little more with pliers or a wrench.

Then have a beer when you're done.

Just visit your local hardware store and the entire thing comes as a kit. Installation often doesn't even require a wrench and you can be done in 20 minutes. Cost should be less than $20.

You may have to adjust the valve to determine how much water you need in your tank.

Also, be sure to shut off the water to the toilet before you start working on it.

parts you are replacing (Home Depot/Lowes). You can replace just the filler valve or "upgrade" everything for that price range. Find a sales associate and ask any questions. Is is not harder than a leggo set. Just turn off the water, and go.