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[
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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.