>>149083655practice. A huge part of charisma is being confident and knowing how to hold a conversation, and the way to get good at that is to just have an absolute shit load of them. If you're college aged or in your early 20s, you can pick up a gig as a bartender. You'll have people constantly cycling in and out and you'll learn pretty quickly how to chat people up, how to read people, how to understand specific personality types, etc. That's a good shortcut since people will come to you and will have a reason to want to talk to you and you'll have a reason to want to talk to them since being friendly and sociable will result in better money.
Otherwise, just force yourself to go to social events and talk to strangers. It will be painful at first, and if your socially awkward, it's going to probably result in a handful of fails initially, but if you treat it as a study and keep doing it, you'll get better.
Most people aren't very good socially, so understanding how to manage a conversation well will make you seem very charismatic because it takes that pressure off of the other person. They get the sense that talking to you is easy because it is.