Before entering too many items for your event, you should set the tax rates that are associated with each item. The choices will appear when the "This item is taxable" box is checked at the bottom of the Item detail screen.
1. Create the desired tax rates
Go to Site Settings > Catalog > Taxes, then complete the form to add or edit a tax rate.
2. Designate Tax rates for specific items
Go to Catalog > Add an Item (or, if item has already been entered, click the edit icon for that item)
Select the tax rates to apply when adding or editing an individual item or use the Bulk Edit feature to apply the desired tax rates to multiple items in a single action.