Hi @Maria915,
Welcome to the Community Center!
To add an extra guest fee, have a look at this Community Help Guide, created by an experienced host.
In terms of changing your cancellation policy, you should be able to follow the steps in this Help Guide.
It's worth having a look at the other Community Help Guides created by hosts, they cover lots of vital areas of hosting. The thread 'What tips would you give a new host' also has lots of good advice from other hosts.
Hopefully you can get your listing sorted and ready to go!
Kirstie