Maybe you need to reconsider trying to become a father if you think pregnancy just happens like this.
You need to make sure you are healthy so you have healthy sperm - that means no alcohol, no smoking, no drugs, no rubbish food. Your girlfriend also needs to do the same, but also be taking folic acid for a while before she considers conceiving.
It takes an average of a year to get pregnant, although in reality it only takes one tiny strong sperm and good timing, it can take some people years to actually get their BFP.
Make sure you consider your decisions strongly and in great detail. Can you afford to support your girlfriend and your child? Is your living accommodation and lifestyle suitable for a little person? Take your time, it's not a race.
If everything right then go for it, and good luck!