You can flirt with literally anyone. I mean, there's billions of people on the planet, so I have to presume you're asking where you find people willing to reciprocate? There's no magic answer. You always risk rejection, but then rejection is usually more about the rejector than the rejectee. In other words, rejection isn't really a statement about the person being rejected so much as it is the person doing the rejection. No one is universally appealing, and so you never know if a person you try flirting with will feel like flirting with you. In fact, most may not. You just keep playing the numbers game until someone does reciprocate. Flirting is also a skill - think of it like a modifier that increases with practice. It boosts your chance of reciprocation but does not guarantee it.
Either you have lived a very secluded life or, more likely, you just don't know how to recognize it. If the latter is the case, then you came to the right place. Unfortunately, no advice we can offer online beats experience. The best advice I can give is to practice, practice, practice, and to avoid taking rejections too seriously. It is BOUND to happen to anyone who practices flirting as much as I used to. It should be more about having fun and less about the results if you really want to excel - that may seem counter-intuitive, but the more you focus on WHY you are flirting, the less fun you have flirting, the less effective you are at flirting. In my experience, you will flirt better if you can steer your mind into a playful, fun-seeking state. When you notices others reciprocating your efforts, then you're making progress.
For the sake of practice so you can get the results you want (e.g., flirt your way to a phone number, date, and eventually a committed relationship or even marriage). Again, focusing on results WHILE flirting is often counter-productive since it puts you into the wrong mental state (same reason you don't look down when in a high place).
However, observing the results afterwards is how you learn. Some tactics will work for you, and others won't. There are almost too many variables to give concrete advise over the intrawebz, but surely there is someone you know IRL who is good at flirting who can show you the ropes?
Anyway, the goal is to flirt your way towards your romantic/relationship goals for life, if any. If you are asexual, then this may not be something that interests you. Otherwise, I would recommend taking it seriously.
When you get better at it, it is kind of fun. I made a game out of it - can I get her number, and so on. I wasn't predatory - just trying to hone my skills so that I would be up to the task when I met the right person. The problem is, if you wait for a guy you're serious about, then you are liable to mess up due to inexperience. Sometimes friendship can overcome this, and sometimes not. Flirting can often take a friend from friend to more than a friend.
Hypothetically, if your life's ambition was to be president of the US, then you would be wise to start by running for a local or state office rather than declaring your candidacy for the oval office. Flirting is similar - don't find someone you really care about and then try to flirt your way to victory with no prior experience or practice and risk losing someone important. Start on someone you don't know instead. If that happens to work straight out the gate and develops into long-term potential, then great. If not, then at least you didn't alienate someone with whom you had a pre-existing friendship.
This is why you start with strangers rather than friends. If it results in embarrassment, then you never have to see that person again. Zero risk. Also, embarrassment is just an emotion that keeps people from their goals in life. Your choices are to get over it or suffer the deleterious effect it has on your life.
Again, if you get to know, like, and care for someone, then this is a bad person to start out with. It is a common mistake. You think it will be easier with someone you know and are comfortable with, but imagine if it blows up on you with someone you know versus a stranger? With a stranger you leave the area and start over somewhere else. With a friend or acquaintance, you can't simply walk away from the embarrassment. Heck, your friends may even know where you live...
When I was a kid, I used to go to an amusement park and practice - lots of strangers in an environment boasting both internal security and entertainment. If I struck out, then I went to a different spot and tried again. I did this summer in and summer out until I was comfortable flirting with women I knew on a deeper level.
Rocco is wise in this. Heed his sage call to action.
