I wish you both much luck and best wishes in your new journey. It is perfectly normal for you to worry about her being picked on. I think most parents worry about that.
While I have not gone from home schooling to "normal" school I assume the feeling you have is much the same as any parent sending their child off for the first day of school. Like you said before you are unable to control what is happening.
Be sure to talk to her every day be it at dinner, or at bed time, or any time you can. Ask her about her day, if she made any friends, if anything particularly good or bad happened. Ask her a million and a half questions. Always check all papers that come home even if it's just a drawing she made during free time. It is important to try to stay as involved as you can. Even if it is just checking her homework and finding out what they learned in math that day.
Volunteer at school whenever possible or at least extend the invitation to her teacher. Do not be afraid to as her teacher(s) questions but try to remember to relinquish control gracefully ;-) Always make parent teacher conferences. If you are unable to meet with the teacher on the scheduled conference days be sure to schedule a date with the teacher when you are available. Always find ways that you can help her along at home such as finding out what areas she needs a little more help in. Teachers are usually very willing to give you extra help on this such as web sites, or activities that can help.
Prepare her for the fact that not everyone will always be nice. This is true both in school and in adult life. I do not know how old she is but there are some great books out there about cliques and bullying. One I can think of off the top of my head is Queen Bees and Wanna Be's. My friend has a 5th grader and they read it and loved it.
Be sure the school understands her situation and are willing to help her along maybe by assigning her a student partner to help her around for the first few weeks. This can also help her to make new friends and feel more comfortable especially since she will someone to sit with at lunch.
Whenever possible reach out to another parent if you have questions. I find that other parents are the absolute best resource when I have questions. I am newer to my school district (coming from private schools) and have learned so much about our new school simply by meeting other parents.
Many schools have family nights or activities (ours do a skate night once a month). We also have a lot of activities at school like plays, fund-raising nights etc. This is where you get to mingle the most with parents. Also, the bus stop can be a fountain of information if you child will be riding the bus.
Most importantly, do not feel like a failure. You tried homeschooling and it just wasn't right for you daughter. I hope that you both handle the transition well! Sorry for making this so long :-)