Sometimes, aquarists add peat moss as a layer underneath the substrate. It will help lower the pH for you but if you are new to shrimp keeping hobby, I would not advise doing that.
You can put some peat moss in a nylon bag and *add it in a filter or just hang it in the tank.
*Warning: peat moss will lower the pH of the water even faster than driftwood. That is why, for small tanks, it is not recommended to filter with peat, as it is hard to control.