Is there a way to straighten my hair without a flat iron?

I have long and very very curly hair which i do not like. I use a flat iron everyday, but right after a shower it goes back to curly. This wouldn't be so bad, but the constant use of a flat iron makes my hair look horrible and feel like a broom. I heard from a friend that i can go get my hair relaxed, which would keep it straight for up to months, even if it gets wet. If I can go get something like this done, is there any damage it could do to my hair/scalp? and about how long does it really last? Thank you.