When starting a foundation, it's important to identify what sort of charitable work you're looking to be involved in. While some founders just know what their calling is right away, it's completely okay if you need to take a bit of time to figure things out.
You don't have to be super particular about it if you don't want to; many private foundations are pretty general about their funding. That being said, most foundations do focus on specific causes, topics, or geographic regions.
Before you make a final decision, do your homework. Research your area to get an idea of what it lacks or needs, and see what other involved foundations are doing in those domains.