The easiest thing to do, at first, is to just find a group of people that are a little stronger/faster than you, and stay with them. If they don't drop you (leave you behind) at some point then they aren't fast enough and you have to find faster people. If they drop you right away, they are too fast and you have to find slower people. Anyway, try to "hold on" to them as long as you can, and keep coming back ride after ride. Hopefully you should be able to stay with them a little longer each time. The basic method of getting faster is to ride faster than you would normally be able to ride, for a short amount of time. This builds up your strength and gradually you will be able to go faster and/or for a longer period of time.

Intervals do the same thing. You ride fast for a short period of time, recover, and then do it all over again. Its best to think of it as similar to weightlifting in the gym, where you do "reps" in a "set". There are different length intervals and different number of repetitions depending on what you are trying to target. A variation is the hill repeat, where you ride up a hill for some amount of time, coast back down as your recovery, and then go again.