Installing WiFi Light Switch - No Neutral
Option 1: Steam or smart switches, cable change

Replace the cable run between garage junction box and switch.  To wire this old school, you need three active wires in the cable: 


neutral (neutral must be white, white isn't necessarily neutral)
always-hot, so the switch has something to switch, preferably black
switched-hot, the output of switch, ideally red 


And voila, /3 cable contains those exact colors.  

When buying cable, look at the breaker on that circuit.  15A breaker: 14AWG is acceptable. 20A breaker: must use 12AWG.  

Option 2: Certain specific smart switches, no cable change

In this scenario, you re-task the black and white wire to the switch to actually always-hot and neutral.   Switched-hot is no longer in this cable. 

At the switch, you install a smart-switch "remote" that is designed to communicate wirelessly or via power-line signaling.  It needs only always-hot and neutral.  In the garage junction box, you install a smart-switch "master" (or faceless module which does the same thing) and pair it to the remote.   The master takes always-hot, neutral, and switches a switched-hot or load output.  

Shopping assistance or product recommendations are off-topic here, so I cannot advise you on any particular models.