Tap your card or PRESTO ticket on a reader each time you enter a subway station and when you board a bus or streetcar. As long as you're within two hours from your first tap, you won't be charged another fare.
If you purchased your TAP card online or by phone, or if you have a Reduced Fare card, your card is already registered. If you purchased your card at a TAP vending machine or TAP Vendor location, consider registering your card online at taptogo.net.