Your profile doesn't have to be long or complex, but it should include your basic information - current and recent employers, industry, interests/hobbies, a brief bio, and links to your websites or anything else you've published online - blogs, articles, etc. Also, be sure to have your photo handy, both online somewhere and on your hard drive, and in a variety of sizes, so you can make it appropriate to the site's profile layout.
Whatever you do, do not set up an essentially empty profile. Most sites will let you poke around as a guest without setting up a profile first, and an empty profile isn't helping you, so there's nothing gained by doing it. On the other hand, most sites have some way of showcasing new members, so if you set up an empty profile, you're missing out completely on the additional exposure you get as a new member, not to mention whatever damage you may do to your reputation. If you've already done this, don't sweat it too much - people are also pretty forgiving. Just get in there and get it done.
While you may have to re-enter some of your data, such as your work history, where you live, etc., most networking sites have a free-form section for your personal bio, networking interests, etc. You can save yourself a lot of time by keeping that information handy in a document that you can just copy-and-paste from. It should only take a few minutes to set up a profile, and even if you don't stay active on the site, with a well built profile, you'll still reap the benefits of people finding you via search for a long time.