Thailand is one of the safest countries in Asia. This doesn't mean that danger is completely nonexistent of course. As in all other parts of the world, there are dishonest people around, but with a minimum of common sense and caution nothing bad will happen to you during your trip to the Thailand.
The number one problems you may face during your trip to Thailand are those of theft, scams and pickpockets. These are especially a problem on public transport and in large cities. Acts of violence and aggression do occur as well, but these are less common. They tend to be limited to certain neighborhoods in, for example, Bangkok and to remote, isolated areas you may visit when hiking. As far as scams are concerned, what often happens is that someone will try to get you to pay high prices for your purchases at the markets or for the journeys you make by tuk-tuk.
Nothing worse than this will ever happen to you, unless you venture into the girl bars. There, you run the risk of having rounds of drinks charge to you at inflated prices or being stripped of your possessions. The again, you'll have no room to complain if this happens. You're aware of the risks when you venture into these kinds of places. The same applies where drugs are concerned. You know full well you're getting involved in something risky.
Another possible concern are the violent demonstrations that sometimes take place in Thailand, most often in Bangkok. These can, however, also occur in the south on the border with Malaysia.
Generally speaking, Thailand is a safe country and all you need to do is respect the rules where personal security is concerned. To avoid attracting thieves, simply be discrete and make be obviously vigilant when using public transport. Keep a good eye on your belongings and your pockets. The pickpockets are particularly adept.
To avoid paying too much for your tuk-tuk or your souvenirs, make sure you know what prices to expect and negotiate firmly, though always with a smile.
As far as the girl bars are concerned, simply avoid going there.
Before going hiking in the north of the country, in the Chiang Mai or Golden Triangle regions, find out exactly what the security situation is. There are sometimes opium traffickers in these areas. If you encounter aggression, do not do anything to antagonize the aggressor(s).
And one final piece of advice, don't get involved in the demonstrations in the cities and never say anything bad about the royal family.