Thailand is generally a relatively safe and easy place for travelling alone. It is a hugely popular backpacking destination, making it easy to meet other travellers when on the road.
As with most places in the world, travellers should avoid deserted areas, especially at night, be careful with expensive possessions, ensure locks work well on rooms and safety deposit boxes, avoid getting drunk when alone, and be cautious of strangers.
English is generally well spoken in busy tourist areas, though language barriers can present a problem when going away from the tried and trodden path. Thai people will, however, typically try to assist when possible.
In common with most countries, there are a few tricks and scams to watch out for. Visitors should avoid gambling and drugs. If somebody says that an attraction is closed, visitors should check for themselves rather than accepting alternative suggestions. Travellers should resist the hard sell in markets and tailoring shops.
Solo female travellers are more likely to attract curiosity than animosity. Questions about marital status, children, and reasons for travelling alone are common and can come as a surprise. Expect probing questions from strangers!