Hello @Ibrahim Burhani,
and welcome to Marketo!
As a first step, have a look here: Salesforce Sync - Marketo Docs - Product Docs
If you follow the steps on the guide you should be all set to use the sync between SFDC and Marketo.
In order to make sure that the leads are visible to Marketo, you can try logging in to SFDC as the Marketo Sync user (so with the user credentials you entered in the Marketo Admin section under the "Salesforce" section). If that user can see the leads, so can Marketo. If this is the case, try updating one of them and see if the lead gets picked up by the sync.
If the Marketo Sync user cannot see the leads, then you may need to adjust your permissions for the sync user.
Finally if you're still having trouble, log a case with our support team who will be happy to take a closer look and help you identify the problem.
//Louiza