@1982luca - I'm assuming you have your paid media tagged (tracking codes), and you've set up your Marketing Channels and related Processing Rules. If this is a correct assumption, the answer to your question comes down to reporting needs and preferences, and either of the following should give you the information you need:
- Use a "First Time Visits" segment that includes only data where "Visit Number = 1" when looking at your Marketing Channel reports
- Break down your Paid Media channels using the Visit Number report to show channel-specific activity based on the visitor's visit number (1 === New; >1 === Existing/Return)
- You might also be able to use the First Touch Channel report, which is populated based on your Marketing Channel Processing Rules, though depending on how your rules are set up, it's possible it will include some number of repeat/return visitors