Hi Beachlover
You will find a lot of information on all of these things here if you browse about a bit. The search facility is good.
Guesthouses. For around 1000 baht you can afford a hotel. Basic rooms are available in the Cafe Royale (1100) or the Ambiance (1200) in Boystown. Both been around for many years.
Koh Larn is definitely worth a visit. It has better beaches than anywhere on the mainland. You can get a ferry for (from memory) 20 or 30 baht.
Bars - any recommendations given now will be out of date by September! They change from week to week. Look around, and find a place you like.
Shouldn't miss. Depends on what you want. I would not miss some of the other tourist attractions - Nong Nooch Gardens, the Million Year Old Stone Park and Crocodile Farm, Pattaya Elephant Village etc. - but thats just me.
Taxis from BKK. I would book one through the hotel or failing that, from Image Limos:
www.imagelimo.com
This is not the cheapest way of doing it - a metered cab from the rank at BKK will cost you maybe 1500 baht (maybe more considering fuel prices since I was last in Thailand) but, after a 12 hour flight its well worth the extra to have someone meet you in arrivals, take your bags to a car and take you straight to where you want to go. I am too fragile for hassle after that flight!
Any beach is better than Pattaya Beach - you will see that when you get there. I personally think some of the small beaches in North Pattaya (sorry - can't give names - maybe someone here can help) are nicer than Jomtien. Koh Larn has the best beaches of the lot.
Anything to watch out for? No more than anywhere else. If you wear expensive jewellery, flash a thick wad of cash and get drunk in bars then you are likely to have the same problems you would in Sydney, or London, or anywhere else for that matter. Don't leave your common sense at home.
Have fun!