Longtime Forza drifter. From Forza Motorsport 4 to Forza Motorsport 6, I completed zero online races, and last time I checked still held I believe 6 individual drift records from various tracks across various releases.
Forza Horizon 4 (All FH actually) use a simplified physics and handling system of the Forza Motorsport series. Drifting in Forza Horizon is like riding a bike with training wheels in comparison. Seeing as you're new to it, I'll give you some tips that others may have mentioned already.
• The beginning cars usually are a bit low on horsepower to drift easily with. So remember to have patience as the more power you eventually get access to by either modding the car you have or by buying a new car altogether, will make drifting much easier.
• The first things that you absolutely need to check are a few settings before you try and drift. Traction Control, and Stability Control must be off, and your shifting method should be manual. You can choose to shift w/clutch, but that's a whole different can of worms so I'd save that for later. ABS can be turned off as well, but it's by no means mandatory. When first starting out, using third person is far easier than using first person view.
• It's important that you know if your car is front, rear, or all wheel drive. If your car is front wheel drive, you'll need to change it. If you can afford tons of horsepower, you can be just like Ken Block and drift all wheel drive cars. It takes practice, and tons of e-brake, but opens all sorts of doors so to speak when it comes to drifting.
• The vast amount of people will probably start drifting using rear wheel drive cars. How easy it is depends on the car, tuning, and horsepower. Drifting via rear wheel drive has a higher point ceiling vs AWD.
• Without literally writing a novel about various tuning techniques, weight distribution, and overall drifting technique. The best advice I can give you is to simply find some open space, and have at it. While it might seem ridiculous, no two people drift exactly alike. So any single person trying to give you a step by step lesson will ultimately result in you beating your head against a wall trying to do something that feels foreign to you. Go watch some Dukes of Hazard, or Smokey and the Bandit, or maybe FF Tokyo Drift, and then try to emulate it. Assuming you've driven a real car before, basic steering into or out of a drift should feel somewhat natural. Starting off in the snow or on dirt can help. Once you can swing your car back and forth a few times without spinning out, move on to pavement. Early on, your best friend will be the e-brake.
If you've got any specific questions, all ya gotta do is ask. Good luck!