I think the best place to start is with learning how to identify, and to understand the structure of, simple noun phrases, and then simple verb phrases.
Once this is done, a focus on the components of sentences. To begin with: delineating subject from predicate. All with very simple models, of course.
To some extent, it does depend on the learner's first language (how similar structurally it is to English) and what he/she already knows (with respect to basic pronouns, conjugations of the verb be, etc.)