When I was in the first grade, I read every book I could get my hands on, even books made for fifth-graders and up. I suppose I owe most of my vocabulary to that. If I came across a word I didn’t know, I would look it up in the dictionary, and I’d read the definition, then I’d go to the thesaurus and look at the synonyms for that word. Say the word I didn’t understand was “brave”. I’d look up “brave” in the dictionary, and read the definition, then I’d put it into my own words. Then I’d go to the thesaurus and find “brave”. I’d read the synonyms, and then I could say “If I run into “courageous”, I know it means “brave”. I know two words now, brave and courageous. I can use courageous to replace brave if I need to.” Then I’d replace common words like “said” or “happy” with “replied” and “joyful”, or “snapped” and “exalted”.
As for sentence structure, I’d suggest using synonyms or “sentence spicers” to add a bit of zing to your sentences. I’d also suggest varying sentence length.