I’ve tried to make this guide as simple as possible, but at some point, if you want to make fine distinctions between flowers, you need to familiarize yourself with the structures of plants and the terminology. This site has flower parts, leaf structures, and more (with illustrations):

Ohio State University Horticulture and Crop Science

This site also has flower parts, terminology and some helpful descriptions of plant families (uses latin names for families):

What Plant Family is it? (Clermont College, University of Cincinnati)

This site discusses the families in more detail, with many illustrations, and photos (although it looks like the flowers are mostly from Montana):

Wildflowers and Weeds