There are a lot of pests that can wreak havoc on your plants, so I’m only focusing on a few major ones. Pests will be the most common for soil growers, though hydro users should always be on the lookout as well.
Fungus Gnats - One or two is fine, and when growing in soil, especially, you’ll see them. Try and keep your soil and coco coir as dry as possible because they are drawn to plant mediums that are watered too often. This is why it’s good to let plants dry out before watering them again. Good airflow will help, as well as throwing up some yellow sticky pads to catch them.
Aphids - These are a type of fly that can appear a multitude of colors though mostly: White, green, or black. Plants become infected whenever a colonizer aphid stumbles on the plant and decides to lay its eggs. They can be difficult to spot, though if you have an infection you’ll know as you’ll see translucent nymphs all over the leaves, with the adults buzzing around. They like eating your plants and then excreting the liquid waste which can cause a type of fungus called sooty mold.
Spider Mites - They are the Emperor Palpatine's of the cannabis growing world. Infamous and dangerous, spider mites will kill your plants. They are the perfect storm of bs because they procreate fast, are terribly tiny but eat/drink a lot, and won’t stay dead.
You’ll probably see their destruction first, which will appear as tiny yellow, white, brown specks on the leaves. If you see those immediately grab a magnifier (preferred) and look for black dots on the underside of leaves where they are hiding. You may also see webbing as well. You need to kill them as soon as possible because they will spread across the plant and to others around fast. This may mean that you need to make a quick decision whether you need to remove one of your plants if it’s majorly infected.
Also, if you have spider mites you can track them to another grower’s room, so be careful and watch when you introduce foreign cannabis plant matter into your grow space. It’s fine if your friend wants to give you a clone to grow, but make sure to ask if they have had any outbreaks, and carefully inspect the plant before you get them close to your space.