Never start online dating with negs!
Here is the deal, if a person is signed up on online dating, it means they are looking to meet new people, hence they lower their bitch-shield (neg works as a false-disqualifier to show ur lackof intrets in order to pass girl's bitch-shield), so a girl at the safety of her home when she signs up for an online dating, does not have a bitch-shield unless you seem creepy or use cocky-funny where its not appropriate, so things to avoid are:
-Negs
-Sleezy comments
-Oridinary compliments (you look so hot)
and ...
I myself do have a gf and I use that as my opener saying
"hellooooooooo
I'm new to this website and just saw ur profile and thought to add ya. Just so you, I DO have a girlfriend -as I don't want ya to think that I'm a creepy guy who farks around with girls- but I love networking and meeting new people.
My name is Shahnam... *then I start a convo about one of their intrests* "
What you need to realize with the opener "im new to this website" is, you need to BE new to the website, as she might have seen you before or you could just say, "I signed up agez ago but never used it, and thought to check it out again and see how it works"
Why this method is good in my view is, first of all, Im coming out as an honest person, who is in a relationship and is NOT trying to hit on them and is just looking to meet new "friends". Plus it shows you are loyal when in relationship, so when u do become single, they would like to date you as they know you'd make a loyal boyfriend!
Now she might ask:
"if you have a girl-friend why you r u on this website? "
or
"if you do have a gf, why do you add girls and wanna meet them instead of guys?"
which to answer to first one, you could come up with diff sh1t such as " My friend asked me to sign up so I can give my opinion on a certain girl whom she's talking to on this website"
and for the 2nd one
"Well my best friends are female and my gf knows it and is cool with it" (Which is actually true about me)
You might ask "as you said Im looking for friends, she might only see you as a friend?" Well that is true in a way, but we start off as "online-friends". we talk on msn and facebook, u get to "listen" to her and help her without judgin her and she will recognize that, so she will trust ya more, and as the reason u said you are friends, is the fact that you have a gf, that makes YOU unavailable FOR HER, and we all know how much we love stuff we can't have! 
Just for the record, I do not go around meeting girls like this and fark around with them when I have a girlfriend, but we all relationships don't last forever, its good to have as many options avaiable!