What’s the best way to change money in CR?

I am from the US and would like to know the easiest and least expensive way of changing money in Costa Rica. Years ago I just used travelers checks, but understand that nobody does that any longer. Should I change some here in the U.S. before I leave? What does one use now instead of travelers checks? Also, will my cell phone work in CR?  Nobody here seems to know. ???? Thanks so much for helping.