Hi There 
I am currently working on a simple 12 bar blues song and doing it in 12 bar sections.
I have the first 12 bars with drums,bass and guitar worked out and with some tweaking here and there sounds great.
What I would like to know is, How can I start working on the next section behind the existing tracks so that when I
click play, it all runs as one?? 
Looking forward to your reply.
Regards.
Bringe
That’s MultiTrack/Overdubbing. Adding one new performance over the old ones (on separate tracks) multiple times.
The piece we wrote is in two sections. The hardware segment is how to get perfect overdubbing with special equipment like a special microphone or USB adapter. You hear yourself in headphones as a perfect theatrical mix just like the audience is going to hear you.
The second half is creating a multi-track show without the special hardware and without the mix appearing in your headphones. This is much more normal if you’re trying to do this with a laptop, microphone and headphones and very little else. The sound delays inside the computer kill you, so listening to your live voice generally can’t be done.
http://manual.audacityteam.org/help/manual/man/tutorial_recording_multi_track_overdubs.html
Koz
Just in case I missed the question, you can make your existing work longer by selecting your music and copy/pasting it onto the end, doubling the length. You’ll have to work a bit to get the beginning and end rhythm to match perfectly. From there you can use the doubling method – select the first two and make four, etc. You probably won’t go much beyond that because you’ll need a break section or other theme change to keep the audience from nodding off.
Koz