Don't try. As we age our hair thins naturally. Some times too much. Go to your hairdresser and tell her to thin it. She can cut it do it appears thinner without damaging your hair. Nothing prettier than long thick hair.
Straigtening it. I have a friend really who had thick hair. And she straitented it all the time, and within a year her hair was like that permanently. Though I don't know how healthy her hair is. Or if you want it that badly.
Use thinning shears, they look like barber shears with teeth. Try taking strands of hair and cut across the strand a few times throughout the length of the hair, I don't recommend taking the hair on the very top of the head/crown because you can end up with short hairs sticking straight up.
You can always wait for old age to do it for you, or if you're in a bigger hurry you can go to a hair salon where they can thin it with special shears. I had this done once, but when the hair that was thinned grew back in and mixed with the longer strands that I already had, I had a head of hair that constantly tangled. So I didn't have it done again.