Snowboard Sizing Guide, Size Calculator
This should help you get your size down. It's not completely accurate, but it's a good guideline.
Like everyone says, your boots are the most important part of your gear. They have to fit just right or else your feet will hurt, they will be cold, &/or it'll be harder to control your board (if they are too big). Try on a bunch and buy the best fitting ones, they don't have to be the most expensive or the most recognizable brand out there. They just have to fit you perfect.
Your board doesn't have to be top quality super expensive, because after 2 or 3 years, you'll probably want something new. So get a good beginner board. Something with a lot of "forgiveness" (like cruise control edges). This will cut down on the amount of times you catch an edge and bail. Also, the longer the board, the faster it'll go and the less control you'll have. Obviously, you don't want this. If you get a board that's a little shorter than recommended, you'll still be able to rip it up and go fast, however you'll find it easier to contol (again, less chances of bailing). Also, having a shorter board is easier to spin when you start trying 180's etc.
Shop the sales(09 gear), don't be affraid to buy used (if in good condition), and don't cheap out on boots.
Start getting stoked, welcome to boarding.