Output deprecation error
In your example, for entry in craft.entries.section('homepage') is attempting to loop through homepage entries as though they're an array. In Craft CMS 3, this is now deprecated just like the error message is telling you.

That means you need to use .all() when querying entries, meaning...

{% for entry in craft.entries.section('homepage') %}


...becomes...

{% for entry in craft.entries.section('homepage').all() %}


Ryan Irelan produced a nice video about this and it's also covered explicitly in the Craft 3 docs. If you've worked with Craft 2 and you're just getting started with Craft 3, that Changes in Craft 3 guide is an excellent place to start.