Go to Component->MyMuse->Taxes to set up taxes.
You can set up taxes that apply to states/provinces or country wide. Whether they are applied depends on the address of the shopper.
A tax can also be 'compounded'. In Quebec, Canada, the Provincial Sales Tax, PST, is charged on the total sale PLUS the national Goods and Services Tax, GST. The second tax applies to the sale AND the first tax. We call that compounded. In this case I would set the GST to be ordered first and set the PST to be compounded.