My car sometimes won't shift out of first gear and makes a ticking sound when it's put in gear

The ticking sound is hard to say, I'd have to hear it. I have a fairly good amount of experience in American & Japanese vehicles, but not much with the Germans. I'd say Step 1 here would be to verify the transmission fluid level is correct, which being an 01 Mercedes, it probably doesn't have a dipstick but rather drain & fill plugs and/or an inspection plug, like how you would check a differential's fluid level. The ticking sound may be unrelated, but I couldn't say for sure.