In SSRS 2008 R2 (hopefully the same for you):
Go to the Design View of the Chart. Right click on the Y-Axis and select Veritcal Axis Properties. In the Axis Options tab there are two settings:
1. Checkbox - Always use zero
2. Text box - Minimum
Note: I just checked straight SSRS 2008 and the difference to the answer is the right click option is titled Axis Properties, not Vertical Axis Properties. Otherwise the same two properties exist in both versions.
Hope that helps, Steve