Everyone says that a cheap one will help you determine whether you like vibrations at all, but I'm not 100% convinced of that -- for me, some of the cheap bullets are painful they're so sharp and buzzy feeling, while vibrations that are pitched lower are extremely pleasant.
It depends on your budget of course, but something high quality and multipurpose might be better -- the Lelo Gigi for example. It's meant as a g-spot toy, but it makes a darn fine external clit vibe as well.
Or, if you're on a budget, there are some pretty useful kits, like the Doc Johnson Black Magic -- three vibes for the price of one, all with similar pitches, because they're the same price range and the same manufacturer, but in different shapes, which is good for experimenting.
Read the reviews, that's what they're for, and you can learn a ton for them. But if you're on a budget, be alert for reviewers who rate down a ten dollar toy because they're comparing it to a hundred dollar toy, whether they mean to or not -- you get what you pay for, but at the same time, it's not really a fair comparison and some of the cheapies (like the Blueberry Bliss for example) are pretty decent bargains.
Just avoid jelly -- it's hard to clean, and could cause allergic reactions. What you'll save in initial cost you'll lose by spending on condoms, not to mention batteries, and you'll eventually have to throw it out no matter how carefully you treat it, because they aren't meant to last, at least in a useful state. They'll last a good long while in the landfill, so they're not the ideal purchase.