Leading you on means he's making you think that he wants to be in a serious relationship with you, but he really doesn't. Signs are: promises he doesn't keep, treating you differently in front of other people, talking bad about you behind your back, interested in you only physically but not wanting to talk about you and your emotions, paying more attention to other girls... basically, anything that makes you feel that he's not really that interested in having a relationship with only you.
If you've noticed something that he does and you're not sure what it means, feel free to post here and we'll give you our opinions.
He's been very nice to me, he's nice to me in front of people, and when it's just the two of us. Sometimes, though, he's mean to a girl friend of mine. I understand why, she completely stalks me, but that's still no excuse. I don't know, he acts like he really likes me, but he... has a wall surrounding him. I don't know if that means that he is not ready, or that he's just leading me on. Any suggestions?
It's very likely that if he has a wall around him that he's afraid to commit to a relationship (which could be for a number of reasons). I would just go ahead and ask him if this is leading anywhere and if he's interested in having more than just a friendship with you.