A bit of everything. Maybe a few days in Bangkok at the start of your trip. Then a train ride up to Chiang Mai for a while. While in Chiang Mai you can take tours, visit Chiang Rai Golden Triangle, Long Neck women villages, etc. Many tourists think that Northern Thailand more represents the real Thailand. Beaches might be another way to vary your visit to Thailand. Phuket is popular with women. If you are a little older also consider Hua Hin. A few days at the end of your trip again in Bangkok. This final stop will allow you to buy last minute souvenirs, be closer to the airport and maybe take in a tour or evening dinner cruise.
In Thailand you can get tours through local travel agencies. Find a tour in the morning and be on an evening tour or dinner cruise that same night. Many tours to choose from. A simple Google check will give you many of the "things to see and do" in Thailand, Pattaya, Chiang Mai or Phuket.
http://www.asiatraveltips.com/ThingstodoinBangkok.shtml
http://www.virtualtourist.com/travel/Asia/Thailand/Central_Eastern_Thailand/Bangkok-1445238/Things_To_Do-Bangkok-TG-C-1.html
Below travel forums can be useful for hotels, currency, scams, safety and how to get from the airport to the hotel. For hotel suggestions mention your price range.
http://www.tripadvisor.com/ShowForum-g293915-i3686-Thailand.html
http://forum.virtualtourist.com/forum-1943-1-Travel-Thailand-1-forum.html
http://www.lonelyplanet.com/thorntree/forum.jspa?forumID=51
For Bangkok if you have a hotel near the Skytrain route you can use that rail service to get around the city. The Skytrain connects to the Airport Rail Link, the subway and with the Ekamai bus terminal.
http://www.urbanrail.net/as/bang/bangkok.htm
If you are young maybe choose the Khao San Road area where other kids go. But no Skytrain in that area. There are cheap hotels and a few hostels in other areas of Bangkok such as Sukhumvit Road (Nana Skytrain) area. The Siam area is also popular but fewer cheap hotels than Sukhumvit.
Good luck.