Odoo's biggest strength — dozens of modules that all connect — is also where most first-time rollouts go wrong. Turning everything on at once usually means nothing gets configured properly.
Start with the process you're replacing
Pick the two or three modules that replace whatever you're currently doing manually or in spreadsheets — usually Sales, Inventory, or Accounting. Get those solid before adding more.
Let usage drive the next module, not the sales demo
It's tempting to enable HR, Project, or Manufacturing early because they look useful in a demo. Add them once your team is actually asking for that functionality, not before.