Most likely, you're going to have to straighten your hair first and THEN curl it (I'd recommend velco rollers for that kind of curl). However, you'll need to get your natural curl out first or else you'll be fighting it. If you don't perform that often, then I'd recommend going to a hair dresser to get it done - the money spent will be well worth the frustration of doing it yourself.

Although, you could not mess with trying to get your natural hair to do something it's not naturally meant to do, by just buying it. Buy a 3/4 wig that's styled how you like & wear that instead!

For one thing, you don't want something curl-enhancing, cuz that's just gonna encourage your natural wave pattern...the best thing to do is start with damp hair, then use a strong hold gel and large, old-fashioned plastic rollers...roll it tight and let it dry completely! Depending on whether you want a tossled look or more smoothness, either finger comb it for tossles, or use a good brush to refine it. I can't guarantee it will work, cuz I'd have to actually see your hair to judge texture and wave pattern, but it's worth a try. :-)