If it is a Marketo form that the lead filled out, then they should be in your Marketo DB (search using their email address). As far as syncing to SFDC goes, unless you have a campaign set up to sync leads to SFDC (like a Lead Lifecycle campaign) then you will either need to set up a campaign that syncs to SFDC when leads are created, or find the lead in your Marketo DB and run a flow action to sync it.
On the flip side, any new Leads or Contacts created in SFDC, that the Marketo Sync user can see, will be created in your Marketo DB automatically.
Hopes this helps.