We also took a bus to a ferry to Koh Samet. It was a nice beach, a great change up from the city.
Learn the train system, it's great. Avoid Tuk Tuks. Expect hassle from the taxi drivers, this was the one thing that really irritated me. Expect to bargain for anything you want.
There will be scammers out there. Watch out - Don't agree to anything that smells fishy. Also, if someone tells you a site or temple is closed, don't believe them.
It's a beautiful city with great culture and excellent food. Be sure you eat some street food.
How long will you be there? Where are you staying? And do you have specific questions? I have a ton of notes about the different areas of the city I used when I planned my trip.