Cathal is right, we need a bit more to help you.
That being said, Marketo does pick up new fields in SFDC very easily. The difficulty you'll have is if you make changes to existing fields. Make sure your Marketo User can see the fields you want to sync (and avoid the ones you don't sync).
I'd take a deep look at the articles and instructions related to the sync.
Data Sync Details
https://community.marketo.com/MarketoArticle?id=kA050000000KzEV
Data Update and Field Block
https://community.marketo.com/marketodiscussiondetail?id=90650000000PSYpAAO
Hide SFDC Leads from Marketo User
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PJxIAAW
If you are running RCM/RCA, you may have to rebuild your model. Please spend some time to plan this redesign and its impact on Marketo.