The first year of an electrical apprenticeship hits you with a lot of new material, and the guys who do well are the ones who study outside of class. Math: electrical math is mostly Ohm’s Law (V=IR), Watt’s Law (P=VI), and basic algebra and trigonometry for conduit bending. If your algebra is rusty, Khan Academy is free and excellent. Practice until you can solve for any variable in Ohm’s Law in your sleep.
NEC fundamentals: you don’t need to memorize the whole code in year one, but you should know how the code is organized. Articles 1-90 are general provisions. Articles 100-110 are definitions and requirements. Articles 200-250 are wiring and protection. Articles 300-399 are wiring methods. Articles 400-490 are equipment. Articles 500-590 are special occupancies. Knowing where things live in the code is as important as knowing the specific requirements.
Hands-on skills to focus on: conduit bending (practice at home if you can get scrap EMT), wire stripping and termination, reading wiring diagrams and schematics, and using basic test equipment (multimeter, non-contact voltage tester). Ask questions on the job - the guys who progress fastest in apprenticeships are the ones who are genuinely curious and ask smart questions.