Definitely go to the clinic. Even if the bike fit is way off, you should still be able to pick up a lot of good general riding tips, skills and confidence.

I need to comment on the saddle position - as a rule of thumb, leg extended but not locked, heel flat. If your toes are pointing down the saddle is too high. Lowering the saddle is for extended descents, descending on technical sections, and dedicated downhill riding. You will have your most efficient climbing with your leg extended, and it's easier on the knees, too. As a rank newbie, don't worry about lowering your saddle unless instructed to do so in your clinic. And no one except crazy downhillers pedal with the saddle lowered all the time.

Irregardless of bike fit, the most important first thing to learn is body position. Probably the biggest difference from road riding is the level of "dynamic" vs "static" in using your body. The #1 thing to learn is that you are most stable with feet level ( 3 & 9) butt out of the saddle, eyes forward, and a relaxed grip. Beginners tend to really glue their bottom to the saddle, and have a death grip on the handle bars. You are more stable in the above described attack, neutral, or ready position because your whole body can act as a shock absorber. You move your weight back off the saddle for descending-this is what a lowered saddle forces you to do.

anyway go have fun and rock those trails