How can I set up a trackpad on a 2017 imac
I already have a mouse, will that cause problems?
No that's fine, it should cause a problem. You can have multiple input devise going at once.
Here is a support article with detailed instructions: https://support.apple.com/en-us/HT201178
Essentially the quickest way is to just into System Preferences > Trackpad, turn on the slider button on you trackpad, and the follow the on screen instructions. You can also hit the Continue button in the lower righthand corner of the Preferences window.