Hi @vmessina - welcome to the community!

If you find your emails going into Spam filters, it could be for a number of reasons - if you think about it from the inbox provider’s perspective (Gmail, Yahoo, Outlook.com, etc) - they face brand new sites sending email every day and have to determine if the emails are legitimate or not.  Sometimes legitimate brands get caught up in this process, unfortunately.  Proven brands will have more data and history to verify its legitimacy.  

Klaviyo has some general overview guides on Email Deliverability that you can brush up on here:

• Guide to Email Deliverability
• Understanding Deliverability Best Practices

A few general questions I tend to find in brand new brands/accounts who are just starting out:

• Are the initial emails (in your Lists) opted in from the website?  Did you import an email List (like your own personal friends and family email addresses) into Klaviyo by any chance? 
• Do you have your List setting configured as Double Opted In?
• I assume you didn’t purchase an email list or participate in some email sharing scheme.  
• Does your email have a good balance of text with images?  Or is it just image-only?  Bigger brands can get away with this, but much harder to do this with a new brand.  
• See if your email has any of these issues: Create Engaging Content

If you let us know about the above, maybe we can figure out what factors might contribute to your emails going into Spam.