I'm thinking of cutting my hair and getting extensions?

Right now my hair is a little under my bust.. but my hair is becoming unmanageable. My straighter has ruined my hair. So I was thinking of cutting my hair to shoulder length and getting extensions. I've never had them before so I need all the advice I can get. I'm guessing I want the glue in extensions. Do you straighten and curl them yourself, or do you buy one set straight and another curly?? Which one's are the best? I've been looking on sallybeauty.com.