Spend some time exploring the forum here...there are lots of great posts with this info. There ought to be some sticky posts as well.
I would not start with a practice test. If you're not familiar with the GMAT and haven't done any prep, you're not going to score well and will needlessly freak yourself out. It won't really be a good score to measure your potential against, you know?
Start by getting to know about the test and then do some practice questions. The best review book that I found was Cracking the GMAT, from Princeton Review. Read it from beginning to end. Also get the Official Guide, 11th edition, for practice questions. There are a bazillion in there. Do batches of each question type, maybe 10-15, before moving on to the next type.
Do a practice test every other week, and thoroughly review the results. Compare them to your untimed practice question results. If you're getting them wrong for both timed and untimed, focus more on the fundamentals. If you're getting them right in untimed practice but wrong on the tests, then work on test strategy (use the PR book for this). Also, make sure you only do CATs (computer adaptive tests), like GMATPrep (official tests) and Princeton Review. I think the
Manhattan GMAT tests are also adaptive, but they're very hard. Kaplan's from what I hear are not adaptive, and do not seem to be reflective of the real GMAT.
Hope this helps. Welcome...and start surfing!