I won't enter into anything specific here. There are many books, courses, sites that are excellent and can help you improve yours skills in working with Access.
As a general guideline, I would say that there are (at least) 5 domains in which you can invest some of your time and energy:
1. Theory: Logic, Algorithms, Object programming, Event programming, Set theory, Relational database theory and normal forms.
2. Access object model: Properties, methods and events of the Forms, Reports, Controls.
3. Data access objects: DAO objects (Workspace, Database, Tabledef, Querydef, Recordset, etc.), ADODB objects (possibly).
4. The VB/VBA programming language and its implementation in Access.
5. The SQL language (at least its Jet subset).