If you are using an emulator called VICE (sorry
if you use some other emulators, I just use this one), then :
Select the FILE menu, then select ATTACH TAPE IMAGE (or just press ALT+T from main screen, it's the same, and quicker!).
Then locate your T64 file, click on it, and select the "Attach" button.
You will be returned to the C64 main screen :
Type LOAD and press ENTER.
Wait for a few seconds (you will see :
SEARCHING
FOUND <game name> ot FOUND FILE)
and when the C64 prompt comes back, type RUN and press ENTER. The game should start immediately.
Another way is : locate your T64 file, click on it, then double click the filename in the "Image contents" window (the filename could be the game's name or just "file" : it doesn't matter) : the game should autoload, without you having to type anything.