Quote from gmst:
If you want to become proficient at excel (not VBA), I can suggest pick following topics and master them. Spend a day or two on each topic.
1) functions like sumif, countif, sumifs, countifs, nested ifs
2) vlookup, hlookup - using vlookup to reference more than 1 column
3) using index(match) instead of vlookup
4) Pivot table
5) array formulas e.g. using sumproduct, sumif etc. in array formulas
6) different number formats especially custom formats
7) plotting different kinds of graphs
8) using addins like solver and data analysis tools
9) filters, advanced filters
10) for big sheets, using auto/manual calculation modes
11) conditional formatting
Above list should be enough - will take around 15-20 days of work. But with above things covered, your excel will be better than 80% of people out there. After covering these, you can start VBA.