I accidentally clicked 'do not trust' on my computer when I plugged in my iPhone and I want to know to to get the option to click trust again?
You should be prompted again if you unplug the phone and plug it back in.
You can also reset your trust settings in the Settings app under General then Reset.
Here's an explanation from Apple's support site:
If you tap Trust, the computer will be trusted until you untrust all computers or you erase the device. If you tap Don't Trust, you'll see this alert each time you connect your device to that computer.
If you're using iOS 8 or later: To untrust all computers, tap Settings > General > Reset > Reset Location & Privacy. This will also reset your location and privacy settings. You can also untrust all computers by tapping Settings > General > Reset > Reset Network Settings. This will also reset your network settings.