Yoko-Gake is a side sacrifice throw, as you drop down you sweep your partner’s foot, falling onto your own side in the process.
Here is a great demonstration on the International Judo Federation page.
Kosoto-Gake is another technique that requires you to attack the foot; this time instead of sweeping you reap the attack, and also, you maintain your upright position rather than falling on your own back as you throw.
Deashi-Barai is a good place to start if you are looking to build up to Yoko-Gake. Learn how to unbalance your partner and sweep out their foot. From this base, you can then go onto progressing the movement into the Yoko-Gake.
If you want to be a great judoka you need great footwork! Good footwork means knowing where to place your feet (or foot) to be in the perfect-on-balance sweet spot for an attack. It also means being able to attack, unbalance and set-up your opponent.
You can get better at footwork by using drills like in the Judo Skill Training book (below) and practicing movement patterns over and over again through Shadow Judo.