Moses gave a really great answer so I'd just like to add on a few concrete suggestions.
Read the news more. Or, listen to NPR.
Start studying a foreign language and learn about its associated culture(s).
Take up a new hobby, something you've always found interesting but never tried, like ceramics, dance, or kickboxing.
Make a list of your goals. For example, some of my current ones are things like: improve cooking skills, dance more, learn French. Once you have set goals and start working towards them, you can become interesting in the process.
Travel more and learn about other people in general.
Really reflect on different issues before offering your opinion. Make sure you're not just parroting someone else's.
Sometimes you already are an interesting person but it just isn't being conveyed because of other factors. It might be your voice or presentation. Try varying the way you speak (pitch, intonation, pauses, gestures, etc.) and see if that makes what you're saying more interesting to people.
Although you might like the idea of being a jack of all trades, try and have some sort of expertise in at least one area.
This is kind of messed up, but I've been thinking that sometimes people that are just *too* nice are boring. It's not that you dislike them ... it's more like you feel like you can't really ever have a conversation with them. Chances are, this isn't you. But maybe it is, in which case you should try to say what you really think about some things, even if other people won't like it.