Online dating isn't dangerous.
However, you will want to know that the person you're with is actually a person you anticipate him/her being, so just make sure that your first few meetings always include you both taking separate vehicles and meeting in populated, public places.
At that point, you know who the individual is, so you run the same risks with him/her that you would meeting a stranger anywhere else.
Personally though, most of the scammers online aren't people who harm you in the real world; they're usually sob story cases from Nigeria, frequently missionaries with severe ailments, who want you to send money out to them. Don't do that. Date people instead. That's what online dating is. Dating. It isn't any more dangerous per capita, or whatever, than any other thing you do where you meet new people and talk or hang out or see shows etc.
Again though... Don't, you know, agree to have a first meeting in a dark alley on the edge of town...
Go to the mall or something.
It also helps if you use a paid site, since fewer scammers/predators will feel the need to pay when there are so many free services (not to mention that paying members are more serious about dating and finding a real relationship).