Hey there @Duradada!
You can access email templates by going to your Shopify admin and going to Settings > Notifications > Customer. It sounds like you'll want to edit the "Customer account invite" and "Customer welcome" email templates specifically. Here's Shopify's general help article for editing notification templates: https://help.shopify.com/en/manual/orders/notifications/edit-template
Hopefully that helps!
@Duradada - you may also consider an app like Spently. Spently helps you created branded email templates, and also include welcome offers in your welcome emails, or upsells in other standard Shopify notification emails.
Spently takes transactional emails on Shopify and turns them into money-makers. Since the free plan is good for up to 50 orders a month, it's a no-brainer to test out.