What size filters do I use for this camera?

I got a camera that's "Nikon D3100" and it's lens' size is 18-55mm 1:3.5-5.6G.
What size filters would I use? Is it 55mm?

NO.

Filter sizes are based upon the lens you are using and the 18-55 mm lens uses a 52 mm filter. That is listed on the lens, usually near the mounting part of the lens

Just look at the front of the lens. Among other things a number followed by mm will be printed there.
That's the filter size. It could be 56mm or 58mm, something like that.

As the others have pointed out, it is marked on the front of the lens. You will see xxΦ (52Φ/58Φ/72Φ), that us your filter and cap size for that particular lens.

Just look under the lens cap of your lens. I'm guessing it's 52mm.

Filter size is not related to the camera. It's the measurement of the filter thread at the front end of the lens.