Have a look at this very good article: Step by Step Creation of Crystal Report using its Features Group, Graph, Cross-Tab and Sub Report
Either, it would be a running summary or a grouped one. Look which one fits in for you and implement it.
For grouping, it's already there in article. For running summary:
1. right click on report click Insert->Summary
2. select the field you want to sum
If it's too complex, use formula fields.