Everyday (one word) is an adjective meaning ordinary.
Example: These are my everyday clothes.
Every day (two words) uses the adjective every to modify the noun day.
Example: I wear clothes every day.
Here’s a quick tip to help: If you’re not sure which to use, replace everyday/every day with each day. If the sentence still makes sense with each day in its place, then you want to use the two-word (every day) form.