What level are you playing now? Are you winning, and if so, at what rate over what sample size?

If you have sound reasons to believe you can jump in at NL25 and hold your own, or if your financial situation is such that you don't mind losing at that level while you learn, you can start there.

In general however, I suggest working your way up starting no higher than NL10, and preferably lower. If you figure you'll still be playing poker many years from now, a few weeks or months that you spent in lower levels than where you could have started aren't very meaningful within the bigger picture.