musictheory.net is a completely free and interactive site
It has lessons and exercises for learning to read sheet music as well as identifying scales, intervals, and chords. I use their exercises for my students in my courses because they go on forever. You can practice as much as you like while getting instant gratification when you get the right answers.
Here’s an example exercise for building chords:
Exercise provided by musictheory.net
It’s a perfect, free way to start learning music theory fundamentals.
They also have a paid iOS app called Tenuto if you prefer apps over web browsers!