My husband and I haven't been married long and can't seem to get pregnant he already has two kids and I'm just 19 what could be wrong?

There could be any number of reasons.
As far as diagnosing it, we can presume, but we can't diagnose. For that, you need to go see your doctor. I recommend you and your husband both going to a fertility doctor. He should be able to determine it.

How long have you actually been trying? Most couples will become pregnant within a year of trying, and some will take longer than that.

As HelpinHere said, it would be a good idea to speak to your doctor about your trying to get pregnant and have a pre-conception check-up. Learn the ways of determining when you are most likely to be ovulating each cycle.

If, after a year (and hopefully it won't take that long) you don't have success, your doctor will likely order some initial testing for both you and your husband. Even though your husband has had children, it can happen that fertility issues develop later.