I've been through that! You are only 18 so you have to realise you do have plenty of time and idealy you should get financially stable first so that you can enjoy the time with your future kids without worries about money
If you and your ex broke up then it obviously wasn't meant to be. I believe things happen for a reason, so in a few years you will hopefully meet your perfect partner and it will be worth the heartbreak that you are going through at the moment.
Why would you want kids? They are not worth it. If you really want them that badly then there is nothing to worry about, seeing as you are only 18 and have plenty of time to acquire the resources necessary to have children.