Where do I attach the Ground and Neutral inside a sub-panel?
In your main panel there's a fat bus bar down the left side. It has a lug up high and another at the bottom (hidden from view behind some wires). Those lugs should accommodate your neutral and ground wires.
Unfortunately... you've kinda got the wrong panel for the detached garage. There's a requirement that all circuits in the building can be shut down with no more than 6 motions - by turning off no more than 6 circuit breakers, in other words.
Usually this is accomplished by using a main-breaker panel rather than the main-lug type panel you have there. You could add a QOM100VHCP or QOM125VHCP main breaker to that panel but it'll be more cost effective to return the panel and buy one that has a main breaker already installed.