I'll tell you my experience. When I decided to have my daughter I certainly wasn't sure I wanted kids. I had always assumed I would grow up and have kids, but was never really a kid person. I wasn't particularly interested in other peoples kids and even found many of them to be annoying. The idea of taking care of a kid was kind of scary and felt suffocating. At the same time, the idea of having a close-knit loving family with children was very appealing in the abstract. I ended up making the decision because the timing happened to be right in terms of career. It was a tough decision because I was quite happy with the way things were at the time.
My daughter is now 4 and not a day goes by when I don't think she is the best thing I have ever done. I don't want to make it sound rosy, it is very difficult, especially the first year, but for me it has certainly been worth it. Imaging a life without her feels very empty.
Just wanted to give you a perspective from somebody that was really ambivalent about children and now couldn't imagine going through life without them. I am not saying having children will be the right choice for you. Having children is radically life altering and I think it can be quite hard to anticipate what the experience will be like before you go through it. The downsides are quite concrete for people while the upsides are more abstract. Personally, I underestimated both how difficult and how rewarding it would be.
Honestly, you probably won't reach a point where you are certain you want children or certain you don't want them. I think you really need to think about what it is that gives you meaning in your life. If you find career personally fulfilling and the idea of family doesn't seem that important, then don't have children. If on the other hand, maybe you enjoy your career, but it is really family and relationships that give your life meaning, then maybe you should have children, even if you are afraid of doing it.