For stuff like flowers and the like, the lens you have isn't bad at all, for a kit lens, actually. You're best off with a moderate aperture (probably around 11, optimally,) and somewhere in the middle of the zoom range. (like around 30-45 mm seems sharpest there from my playings-around.) You should find the lens allows you to physically get pretty close that way. While you're just starting out, I think the kit lens is enough of a 'macro' for some fun and learning. Not enough to satisfy the serious macro shooters (There are a whole lot of really good macro folks here: I'm not one of them.

) but I was surprised to find how useful it was. Considering my kit lens actually came free, and how back in the day a kit zoom was a 'macro' only in token, I consider it pretty outstanding for a start.
So I suggest you don't feel like you have to buy something before you start playing/learning. If you get really into it there are plenty of ways to improve your results.