Most people who date online have no problems, but there can be dangers and you have to be sensible.
The main dangers include identity theft and fraud - these can happen even if you haven't actually met the person - and if you do meet, of course there is the chance of a violent or otherwise dangerous person.
Regarding identity theft, it's very important that you don't give out any personal information to someone you don't know. Anyone who tries to get information out of you like your home address, private number etc may be up to something - genuine people who use online dating services will respect privacy and understand if you don't want to disclose information, so be very wary of anyone who seems to want to invade your privacy straight away.
The commonest type of fraud is where someone starts asking you for money very early - usually because of an 'emergency' or a 'business opportunity'. Never, ever believe any stories like this.
Finally, if you do decide to meet face to face, you must tell someone where you are going, and arrange to have the first meeting in a public place, not near your home.
If you are careful, online dating can work out well. You can get some useful hints here.