My youngest is starting in Tues 
His sister will be in Y9. Fit some reason I'm way more anxious this time around.
When my dd went, she knew no-one I I wasn't worried thinking it wouldn't be a problem. It wasn't a huge problem, but it was waaay harder finding a small friendship group than I thought it would be. Alls good now though.
Even though I think in some ways my ds is slightly more confident than my dd I still worry.
My tips would be -
Join in as much as possible! Joining a Y7 lunchtime Drama club was a lifeline for my dd.
Help them check and organise their bag for the next day. After a few weeks they'll do it on their own ok. Same with homework.
Ignore all the Facebook posts from other parents (even at different schools) after the first day declaring that their little x is LOVING it and had taken to it like a duck to water.
These used to upset me the most as my dd was taking her time, having her wobbles. I felt really alone thinking I'd got it so wrong. Now 2 years on, those parents tell me that Y7 was very up and down and the first 6 months really hard. I wish I'd known that then.
Tell your dc that it looks as if everyone else is super confident and have a hundred friends already but often many children are feeling the same. This nugget was something my ds's form tutor told me when I wanted to chat about ds's settling in. I think it's very true.
So my biggest tip is .....time. Give everything time. Don't expect brilliance or immediate happiness instantly. It will come.
I'll try and tell myself this next week.