I have just done a few upgrades but can only give you advice as I see it.
Firstly, don't arrive under prepared. Most people have years to learn their company manuals and SOPs. A lack of preperation stands out!
Next. If you are being put forward then your company thinks that you are up for it. Dont worry about your ability, think about what's going on in the aircraft. Lots of people put on blinckers and fail to see the overall picture. Deligate, observe, help and praise (or encourage).
Finally. Try to avoid being everybodies best friend. Set high standards for yourself and you will find others follow. Don't be afraid to ask for advice or help and, as has been said before, don't try to reinvent the wheel.
Good luck.