There are many websites including =one by the Tourist Authority of Thailand (TAT) that deal with first-time must-sees. A good taste of Thailand, IMO, can be had by:

a) The triumvirate of Temples (Wat Phra Kaeow, What Pho and Wat Arun)
They are all clustered around each other (Wat Arun being on the other side of the river) and are a good insight into the history, culture and architecture of Thailand
b) The Golden Palace
c) A Sukhumvit bar or Patpong to see seedy nightlife Asian-style
d) A water taxi ride to see BKK from the river and canals.
e) A ride (hopefully short one) in a open, 3-wheeled motorcycle taxi (tuk tuk)
f) Kicking back at the Oriental Hotel and enjoy dinner with a cultural show at the Sala Rin Naam
g) Shopping at Siam Square to experience local BKK metropolitan life
h) Taking in a Muay Thai kick-boxing match
i) The Jatujak weekend market
j) The Ratchada HwayKwang night market in Ratchada (very local indepth experience-hardly any tourists)
k) Food, food, food from sidewalk hawkers to tony establishments.