People liking us or not is often a reflection of how we view ourselves. As to whether we are likeable or not.
I assume people are going to like me unless I give them reason to do otherwise (which I generally do not do so people not liking me is usually not a problem per se unless they don't like me for reasons that I am unwilling to do anything about such as my stand on doing the right thing by God for example).
So the first thing would be to think about how one views oneself and to change that if it is not so good.
Secondly I would focus on being a friend to people who are less likeable than me. So go find the homeless, the disenfranchised, the social outcasts if you will...and befriend them. Show them kindness, invite them to lunch, talk to them about what might interest them, etc. It will do wonders for the view you have of yourself and to the degree that you learn to be unselfish in your dealings with others it will actually make you a much more likely fellow (or gal).
I personally can't stand being around people who are always talking about themselves. So don't be that. Be the opposite and become almost instantly likeable.
Which brings me to another point. An easy way to be likeable is to simply master the art of getting others to talk about themselves. Striking up conversations with strangers on public transportation is a great way to practice in that if you mess up...no big deal, you will probably never see who you messed up with again.
It's really easy to get others to talk about themselves. I do it all the time and find what people tell me to be generally pretty interesting (once you get them to talk and find an area that they feel strongly about).
Just find something to comment on about their person or what they are doing. Do they have unusual looking sandals? "I've never seen sandals like that before. Do you mind if I ask where you got them?".
How about a bunch of shopping bags? "You know I used to find it such a hassle to carry all these bags everywhere. I started using a cart I bought at a flea market instead. It helped a lot (and then shut up and let them respond if they care to)".
Nice behaved children? "I hope you don't me saying so but I couldn't help but be impressed with how well behaved your children are. Most kids can be so unruly (then shut up and let them talk if they care to)."
There are a hundred different things you could say to get others to talk about themselves.
Sometimes it's good to say something about yourself as an opener. Like "I hope it stops raining (while looking out at the clouds)"...again shut up and let them respond if they care to.