Very common question. Do a search on this site, and you'll find more answers. Until then, it sounds like your IACV ( Idle air control valve). Coolant runs through it to stablize your idle. Number one thing to do is to check your coolant level. Low coolant level will cause an insufficient amount to flow through it, causing the up and down idle. If that doesn't solve it, it could be clogged. Hopefully it's not. check your coolant level and get back to the forum. There are a few more things you can do, but try that first.