You should prepare your SFDC to be Marketo friendly before you sync it. That is, think hard about your lead lifecycle and make any adjustments within SFDC now.
Then make sure your SFDC Marketo User has the right permissions to view/change/create records and fields you want.
Then go through the integration guide, step by step. I also have an overview here: http://www.marketingrockstarguides.com/marketing-rockstars-guide-to-marketo/