A 10-16oz or 50cl shaker should do you fine if you're making normal sized drinks.
However, since you're new you may want a little wiggle room in case the drink is off (say, you need to add more of one of the ingredients to make it taste like you prefer). So you may want to consider a larger shaker just in case (plus if you or your friends like bigger drinks, might as well make it only once).
I'd recommend paying no more than $20 for a shaker unless you're getting something fancy. Here are a few decent links:
These "should" fit on most any pint glass and are much cheaper:
http://www.equippers.com/shop/product-d ... iid=365315