I dont know of many surfboards for sale on this site
so I guess Ebay and Gumtree and local surfshops are your friend. EDIT: Actually there are heaps..see my next post for just one example
I would be suggesting around 5'8" to 5'10"..also with Firewires and similar epoxy boards you can go up to 4" or so shorter than a fibreglass board due to their higher bouyancy and flotation....perhaps even the local shaper sometimes sells 2nd hand boards...also Christmas is coming up...try and be really good and Santa (mum and dad) may help....perhaps if you agree to do some extra stuff at home (wash cars, mow or whatever) it could help too...Strike a deal with them that if you come up with half they front the rest...basically suck up big time over the next few weeks.
There's a guy over your way who sometimes puts STEALTH Carbon Fibre boards on Ebay with low starting price...I picked up a 6' for less than $500 and that was shipped to Sydney...They are a popout type board but ride very well...just remember to keep Epoxy and Carbon Fibres out of the sun as much as possible as they damage from heat much more than trad fibreglass boards.
If going for 2nd hand...look for dings which may not be fixed and not too many impressions on the deck (tho if it has dings better in many ways on the deck than underneath)...also perhaps a trade in on old board?
Ask your mates as many questions as you can about their likes/dislikes...initially they will probably say that their board is the best but if you ask the question like what are the 2 or 3 things you like and 2 or 3 things you dislike you may be able to work out what suits you....look at some of the better surfers that are similar in size and see how they ride etc...but if your level is way below theirs be aware they could be riding a super thin/light stick that may be too advanced (Sorry dont know your level).
Finally, keep perstering the local surfshops....if you keep popping in without being arrogant or annoying..who knows they may even offer you a part time job.
Just thoughts....I remember when I was your age (long time ago) I picked up my first fibreglass board which was a reject from my uncles quiver...not the best but a good learning board. Tho I surfed on a "foamie" for 6 years before that as boards were way more expensive (comparatively) then than they are now...and we were "Dirt poor"

BTW...firewires can be overpriced...but keep checking Ebay....and if you see something you like...dont bid on it immediately...its like playing cards...dont show your hand until the last 10-15 seconds (if its still within your budget). Make sure you ask them questions about anything if its not described well enough. Set yourself a reminder of the time/date and logon 15 mins before the end of the auction to see how it goes. IF a board doesnt sell cos its a little too expensive asking price...contact the seller and try offering him a fair price (same as Gumtree....I got 2 softboards for my kids that way). GOOD LUCK