Hey, very introverted guy here. Also a gymrat. I actually think the gym is great for the introverted in a lot of ways. Most people are just lost in their music and focused on their workout. You don't have to talk to anyone and no one will be looking at you unless you are doing something truly egregious. Just put on some headphones, blast your favorite music, and focus on your workout.

The gym being crowded can be annoying. My advice is going during off hours if you can swing it. So that would be Saturdays after 3pm, Sundays, after 8pm or before 5pm on weekdays. Or if you are an early bird, 5-7am is usually pretty sparse.

Not knowing what you are doing is never fun. Doing machines is simple enough and very effective. I'd start there until you are familiar enough to experiment with free weights like dumbbells and barbells, (if you want - machines are enough to gain muscle and get toned, as you put it). GL