Office365 and SharePoint advisors please

We've just endured an 'update' from a hosted desktop (WPL) to O365+S/P.
Putting aside the horrendous bandwidth issues the other office is having and the constant stuttering/pausing at our office, it's starting to come together.

So, how do I make use of this thing?

What top tips have you got?

What crap do I ignore for a company of less than 20 - I've already had a colleague keen to message me all day on Yammer.

We're not going to use the OneDrive - all our company files are on SharePoint and we want to avoid data duplication and isolation.