My sister gave me her old iPhone s and she forgot the password how do I delete everything and start fresh

Ask sister to see -> See this Apple article: If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

Then she has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Then she has to remove the iPhone from her account, see this Apple article:

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to see where you're signed in - Apple Support

Then you can sign in with own your Apple ID.

If it is just the screen Passcode, follow the instructions in the blue link above your name on this page which will erase the phone so that you can start fresh. If it is Activation Lock, you will need your sister's Apple ID and Password to get into the phone. Without that info, you will never be able to use the phone. Activation Lock is a theft prevention feature which cannot be bypassed.