Hair Qustion: Men straightening hair?

Hey all, I have thicker hair. Now my hair is not a poffball, but it is thick. I want to style my hair differently but the thcikness of my hair severly limits what I can do to it. So, my question is how should I make it thinner. My girlfriend told me to straighten it and so did my mom. How long would it take per day for a guy to strighten a mildly short haircut. And would this make it appear thinner? Also, does it actually damage your hair? My gf and mom said it used to but not anymore because of new advancements in conditoners, etc. Please Help! Thanks all