Accept that the first day, and probably a few days following that, are likely to feel a bit 'emotional'. It happens to everyone. It will get better from there quite quickly, though will probably take at least a term before you really get into the swing of things.
If you haven't done so yet, try going to the admin department and seeing if they know what courses you'll be doing. They're useful people to know in general, and I've found them often to be much more on top of things like this than the faculty. The same applies to lab technicians and other staff (who will absolutely know a heck of a lot more about this stuff than you do, so stay on their good side).
If you're not already, get familiar with the way things are generally 'done' at your uni. With undergrad courses and practicals there's quite possibly a standard way all work is assigned and graded. Look in the undergraduate prospectus and/or any webpages set up for them to get an idea what they're being told, and hence their expectations. Also look up any rules your department has on plagiarism, dealing with late work, dealing with sick days and religious holidays, what constitutes appropriate and inappropriate behaviour towards them on your part, etc.
You will almost certainly need a calendar to keep track of when you're meant to be doing what. I've used a pocket book in the past, but these days would probably use my phone so I can sync it all online. Being organised, and knowing that you're organised, removes a lot of the stress.
Once you find out what course you're doing be sure to go through all of it. If there's a practical demonstration, do it yourself as soon as possible - and once you can do it, play with the equipment and find all the ways it can fail to work (your students will discover them all). If there's questions students will be expected to do, do them all yourself, no matter how easy they look - you need to know this material better than they do, so you can deal with the thousand and one bizarre errors they will make. This will be a bit of a time sink, but you've got to do it.
Be prepared to not be surprised if you start to find you're enjoying it. Even if teaching isn't your long term goal, seeing someone 'get' a problem thanks to your advice can be very rewarding.