Get to the classroom at least 15-30 minutes before the day begins and you have to pick up the kids or open the door for them. This way you can go over the lesson plans, check the teacher's guides and get an idea of what you have to do with them during the day. If you dont' understand something then ask one of the other teachers nearby (usually next door- same grade) if they can help you with what you dont' understand or are unsure of. Even in 1st grade you will have students who know how things are done and who will help you. There are ALWAYS a few who will be helpful, especially at this age. Have stickers for those who are on task and praise those who follow directions. "I like the way 'Johnny' is sitting quietly", "Thank you, so-and-so for doing such and such". You will be amazed at how fast little ones, even the boys, will settle down when they see rewards (even as small as stickers) being given out. Remember that young ones have very short attention spans, especially when it comes to sitting still listening to a story on the floor or working on things. If they get too antsy you can always stop, have them all stand up and get the "wiggles" out. I have done this will kids of all ages (up to 6th grade). Have them stand up (tell them they have to stay in their own space), then give them commands like " stand on your tippy toes, reach for the sky, wiggle your fingers/hands/arms/shoulders/body/legs/etc.." and be sure you do it with them so they see how you want them to control themselves. Then after they have wiggled it all out tell them "now sit down". Usually kids flop down and you can go on with the lesson/story. 10 minutes is usually the longest (if they can make it that long) that young ones can sit still. Keep instructions short and simple.
As for the bathroom, if you have a lot of kids asking to go at the same time, then take a break and walk them all to the bathroom together as you do not want to say no then have one have an accident. Have them line up and let 2 boys and 2 girls go at a time, when one comes back allow the next one to go.
Go over their classroom rules and let them know what you expect from them right from the start. One trick for the bathroom too is to walk them to the bathroom before you let them go out to the playground at recess or stop at the bathroom on the way back in. One of my rules was always "Go to the bathroom during recess" and "if you ask to go the bathroom within 30 minutes of coming in from recess then you own me time". This usually works unless there is a child with special bathroom needs then they get to go whenever. The teacher should leave notes of who are good helpers, who goes to other classes /gets pulled for extra help/etc... and anyone with special needs (bathroom, office for meds, etc...).
The big thing for a lot of teachers is to get as much done as you can during the day. Most I know put down more than enough stuff to do to keep them busy. If you find that you have extra time at the end of the day, then you can read stories, take them out to play, etc... Just be sure you leave enough time for them to clean up at the end of the day and to pass out anything that needs to go home (HW, notes from the office/teacher, etc...).
Most IMPORTANT thing......... Leave a Note!!! I know many subs who have not written a note at the end of the day or who have not notated what they got done on the lesson plans itself. I tended to do both. I would check off what we got done in the allotted time and note why we didn't get something done. Sometimes I left a very detailed note and other times i kept it short. If i knew that they would be having a different sub the next day (if i was unavailable for multiple days) then i would leave a detailed note to let them know what we did, where we left on something (so they could pick up with it the next day) and any other information I thought would be helpful. Leave the names of students who were good and who caused problems as well as what you did/the consequences so the teacher hears from you about any incidents.
Kids are notorious for saying "but Mr/Mrs/Miss So&So lets us do this..........." Just follow your gut and if it's something you aren't sure about, play it safe and just put it simply to them "well, you can do that when Miss/Mr./Mrs. comes back/is here but/and this is the way i want to do it".
Good Luck and HAVE FUN with it.