Purchase the best instruments and microphones that you can afford. They don't have to be mega-star quality, but starting out with beginner instruments will make you end up with a lackluster, childish sound. Purchasing instruments secondhand can be a great way to save money while getting higher quality.
Practice your sound. While you might be a natural at singing or playing, you probably aren't a natural at playing as a group--or at sounding completely professional. Regular practice sessions can keep everyone sounding in tune and more experienced.
Invest in a mixer. If you are planning to ever have more than one or two tracks in your song and you want to sound professional, a mixer can allow you to record different instruments on different channels. It can allow you to mix them together later. You may not need to spend a lot of money on this piece of equipment, depending on how heavily you are planning to rely on your computer for mixing and editing.
Buy some good music software for your computer. If you use a Mac, you may already own some powerful software that comes with your computer--GarageBand. Software such as this can allow you to add effects to your vocals and other tracks, place samples and line up tracks that might not be on the beat. You can also enhance your sound in a number of other ways.
Be professional about your output. In order to sound like a pro, you will need to put out an end product that is professional. Burn a compact disc of your music as a master. Think about getting them professionally copied, especially if you are planning to sell them.