Agreed. If you have never gone before you need to rent first. You can also try out different sizes. If you rent once and get a 159, but you want to try a 156 the second time, just tell them at the counter and they'll get it for you.
Also, before you buy, I recommend you demo some stuff. It's worth it when you consider what a board costs. Mainly, try different flexes. Since you didn't mention a style, I can only assume you aren't sure yet. That factors into your board, too. You don't want to buy a park board only to find out you hate the park.