Hey, I left Windows platform quite a time ago. Now that I'm getting back - what should I do first when I get the phone? Get to insider program or stay on officials? Install some specific apps for phone? And what are the things I should be prepeared for?
You can stay on the official build as it is quite stable. Currently the insider builds in fast ring do have a number of bugs that could compromise your experience
When setting up the phone, use wifi and keep the phone charging and update the OS. Don't download any app, don't copy over any data. Just update the OS, then after updating the OS update the core apps (check store app an check for updates).
Once OS is updated, core apps are updated, you can then start installing the apps you need, syncing emails, copying over content etc.
Do all this while it is charging. Once all your apps are installed, head over to settings> system> battery > battery saver > check which apps can run in background and disable the ones you don't need.
As a recommended solution for battery drain, sign out of skype apps
What you can be prepared for : try to make sure that you install any updates while on wifi and charging. If you keep wifi or cellular on things could update without you knowing and hence drain the battery fast. So disable automatic updates