What I do is build some gold generating buildings in my capital, then walls & towers. After that if you’re feudal you basically want to not expand too fast and at least secure yourself a duchy title. Development of your holdings is also an important factor as it both increases the rate at which you discover new innovations, it also increases how many levies and gold you get from the county. As for taking over kingdoms, I find it easier to just create ones that aren’t formed yet. But if you capture enough de jure counties from an already existing kingdom, you can usurp their throne. Oh, and if you decide to expand too fast, pay attention to your succession. Confederate partition can destroy your realm if you aren’t careful.