How can i make my hair thinner and straighter?

i have mid-length wavy hair that does get quite curly and frizzy. i straighten it every morning, but as soon as i go outside it immediately goes all curly and thick! so i was wondering if there are any ways to make you hair straighter + thinner, and so it gets less curly when you're outside.
i am thinking of going to the hairdressers, if i do what should i say to the hairdresser if i want my hair to be thinner and stay straight for longer? thanks everyone :)