Is it werid wantjng another baby and you already have an 18 month old. I want another but im only 18?

Not weird, but... No, it's not weird. Babies are great, and having one often makes you want more. But, make sure you stop and analyze why you are seeking this out. Are you in a stable relationship? Are you trying to fil something that you are missing in your own self worth? Are you using the baby as a way to alter or solidify a relationship. Only pursue this this soon, if you are stable, you can afford it, etc...
Can you? You may very well be the motherly type. Being this young does not make you any less of a capable mom. However, do you have any feelings about what your other 18 yr old friends are doing? I'm sure by now you know how life changes after a child. Having a strong support system helps. So please think this through.