Welcome to the team!!
First of all, as far as restroom breaks go I would make sure that you tell your store manager privately that you will need that sort of accommodation so that she can let the shift managers know. If you don’t feel comfortable with that, that’s okay too! At my current location, any time you feel the need to go you simply inform the shift manager that you need to “take a personal” and they will have someone take your place while you go. This can be difficult during peak especially, but if you’re at a low volume store it shouldn’t be a problem.
As far as the dress code goes, again it can really depend on the store manager. Mine was fairly lenient when I was first starting. I would definitely invest in nonslip shoes; I have seriously injured myself because of the slippery floors. The floors will be wet. Always.
$1 is a wonderful resource for this; I got my current non slips for $40. Otherwise, just make sure you wear modest clothing. Stick to black, white, gray, and navy until you get more familiar with the
$1 Don’t sweat it though! You got this, just remember that lots of people less capable than you undoubtedly work at Starbucks. Ultimately, it’s just coffee. You’ll do fine.