Around Town in Pattaya
Pattaya is a beach town, dress is informal and a matter of taste. For shopping in town, shorts and a top with sandals are fine for both sexes. You will see men without shirts around town but most Thais are uncomfortable with that.
For dinner out in the evening, your chosen restaurant will set the standard. Up market restaurants prefer customers to dress smart casual -- long trousers and a shirt for men and slacks or a skirt, and a blouse for the ladies. Other restaurants assume a "dress as you please" attitude since they all seek your custom.
At the Beach - Pattaya or Jomtien
Both sexes must wear a bathing suit as a minimum; Thai nationals (both sexes) will generally wear shorts and a shirt, lady tourists rarely go topless. The Thais will tolerate toplessness from tourists but they don't like it.
Hotel to beach, wear something over your bathing suit. A short sleeve shirt, blouse or tee shirt works, as do tank tops.
Clothing for Visiting Temples (Wat)
Err on the side of conservative. Otherwise entry may be refused. Visitors to a temple or shrine should cover-up. For men this means long pants and a sport shirt. For women, a knee-length skirt or slacks and a top that covers the shoulders.
Sandals are fine because you remove your footwear before entering the sacred areas.
Clothing for Immigration Offices, Police Stations & Government Buildings
Probably nothing will be said to you about your attire but your respect to all Thai Authorities is best shown firstly by dressing appropriately. Long trousers, office shirt, shoes and socks are necessary for men and similar smart attire for ladies.
Warmer clothes for the evening are not necessary since the evenings stay very warm. Dressy shoes are really out of place unless you are staying at a 5 star hotel and/or going dancing.
Lightweight, good value clothes can be purchased locally from department stores and street vendors. This includes bathing suits, lightweight tops, shorts and sun-protecting hats.
Laundry services abound, all hotels and most other accommodation provid excellent service. Do-it-yourself laundry of "smalls" is practical in your hotel room since these will be dry to wear in the morning.