I like her but she is in a relationship.
Are you trying to get her into a relationship with you instead?
I'm trying to get into a relationship with her.
Even though you might be interested in dating a person that you like, it's very wrong to try to break up an existing relationship for your own gain. It would be selfish and wrong to do so. You should try to find someone else to date who is single and available.
I really want to date her.
It's rude to pursue someone who's not single. And if she's willing to dump her current boyfriend, she's likely to dump you, too.