The sandbox is very similar to Marketo production environment. The main difference it that I believe there is a low cap on the amount of emails that you can send from a sandbox per day. This is because your sandbox is meant for testing not for marketing.
If you have synced your sandbox to an SFDC already then that's fine. Ideally you will have synced your Marketo sandbox to an SFDC sandbox. Once you sync a Marketo instance to an SFDC org they are paired for good, it's not possible to switch your Marketo sandbox to sync with another SFDC org. It's ok to refresh your SFDC sandbox but you will need to contact Marketo support to help you to re-establish the sync after the refresh is complete.
Let me know if you have any specific questions.
Rgs,
Cathal