I agree with ericm. As a beginner the best thing you can do is ride more. Just keep going farther and faster and you'll improve. Just make sure you have enough water and calories (people exaggerate the importance of where those calories come from too IMO) during your rides and you'll be fine.

It also helps to find a group to ride with, especially one where you have to work to keep up (but aren't getting completely dropped every ride).