Wear clean clothes according to whatever dress code they prescribed (when I was in LA, all the ride alongs were supposed to wear white collared shirts, polo style, dark slacks, black shoes, preferably safety toe boots if you have 'em). Show up early ready to help do shift change checks so when shift officially starts you're ready to roll right away if needed. Bring something like a box of donuts or ice cream or whatever with you. Bring some sort of EMT study book so in between calls and other downtime when you're not talking with the crew or helping with chores or whatev you can be doing something more productive than playing on your phone.
Be prepared to act within your scope/level of training (especially if they also run your training they should know exactly what they want you to do... whether that's a near graduating EMT doing full BLS assessments or just a Day 1 (day 0?) student only watching/listening, even then you can still be expected to help lift/carry and maybe CPR if needed. Be sure to ask the crew you're working with before you actually get a call. In my experience it's common for the Student ride along to just watch the first call or two and then be slowly eased into whatever role, but be ready for whatever you've learned in class.
Be calm, polite, courteous, treat everyone like they're your future boss, don't talk smack about anyone, even if it's a troublesome patient the crew is b***ing about later don't join in, especially do not do that for other crew. Time for 12+ hours of perfect customer service mode, save venting for after you get home.
But definitely, enjoy yourself.
