It is so hard to find a boyfriend or girlfriend.
Regardless of if you know the person or not, the best way to flirt and or start a relationship is to be yourself. A lot of times if you are yourself that is what is most attractive. If you are out in a club/bar/outing, smile at the person you are interested in. Keep and eye on them (not in a creepy stalker way), but let them know you are interested with your face and eyes. Don't make the other person jealous too much. This may lead to them just thinking you are trashy and a tease. You have to keep it under control.
A way to flirt when someone you are already with is to have fun.
Smile and laugh with each other. Rub his/her arm when you are watching TV. If you are out to dinner, rub your dates foot/leg under the table. When you do that, give a subtle wink across the table or a half smile. This will make the mind wander and think about you , big time. Keeping a little bit of mystery between you will keep things a little more exciting. Don't spill everything about yourself when talking about the evening. Leave out some details that will make him/her wonder and want to ask questions about what you are talking about.
Have a conversation with the person; just talking to someone is a good way to get to know someone.
Flirting does not also have to be bad. Just talking is awesome and a good way to build a foundation to a relationship. When you talk to someone you really can tell if it is someone that is worth pursuing. Above all respect each other and have fun.