A Mac system generally consists of no more than four passwords for general functioning:
1. Administrative password -- this is to install software and log you into the system or make changes to the system. Normally this is only needed when doing those operations.
2. A Wi-Fi password -- not needed if you're not using Wi-Fi, but who isn't anymore?
3. An Apple ID/iTunes password -- based on your present email address. Not strictly mandatory, but terribly useful.
4. If you're also setting up a new Wi-Fi router, you'll want to put a password on its settings so that nobody else can change them.
As for whether you're better off just erasing and starting over, the answer is "depends." You didn't mention what kind of MacBook or how old it is. If it is one that is a few years old AND came with the system and apps installation on DVD, then yes, you can start over fairly painlessly. If it didn't, then it's probably best to just stay with what you have set up.
You can change most passwords fairly easily. They are located in Keychain Access, and require your admin password to access. Should you wish to change the admin password, have a look at
this.