Where is the best beachfront location in the Cape Cod area?

I like crashing waves and lively beach communities, fishing boats and piers where fishermen get up early and head out for the day's catch. My wife, some friends and I are thinking of spending a week on Cape Cod this summer. But we've never been, and I have no idea about where we should stay. We're not looking to break the bank, but we have a little money to spend if it gets us in the right place. Where should I look for a vacation rental cottage or condo that is within walking distance to a great beach?