From my own experiences over the last 10 years...
1. If you're going to do it don't use a free service.
The fact that you have to pay money to contact someone roots out about 90% of the time wasters.
2. Some sites are really just show off sites. Like our Adult Match Maker...
Lots of the girls who have dozens of
sexyish 
photos and it soon becomes obvious they have no intention of actually meeting anyone.
3. So pick a site which is about real people looking for a real connection. I can't recommend any because that's something you have to find out by trial an error.
Locally we have Lava Life and RSVP. Both pretty good sites.
ALSO... Be warned. There are some really damaged people on-line who are not ready for anything.
I'm talking about cancer survivors, car crash victims or survivors of serious illness.
Now I would not reject a lady because of these reasons but again they are simply not ready for anything.
And another thing... Promiscuous women are to be avoided at all cost. It's simply not healthy and always more trouble than it's worth.
One last point. Like most things in life it's a numbers game. So contact a lot of ladies and see what happens.
Don't let yourself be put off by rejection. It's not personal. I know that easy to say but after a while you'll get used to it.
I have a nice lady in my life who I met through a site. She's sweet and caring and she even has a good job!
She's a breast cancer survivor but that was 10 years ago so she has dealt with it and is now doing fine.