Illustrator Help

Ok, so I’ve had illustrator on my computer for years, and never really used it, I just stuck with Photoshop. I opened it up today, out of boredom, and am having troubles with the basic paint and pencil tools.

When I draw it makes the line that I drew with my Wacom pen, then when I draw over it seems to erase parts of the line under it, and if I start too close to the first line, it just changes the shape of the line altogether. Is this just what illustrator does? Or is there a way to turn it off so I can sketch undisturbed?

Thanks,
3dnirvana