Enter competitions with your horses
Vote in the Creation Space when you have unlocked it
Breed and sell horses
Sell items you don't need

Another tip: Don't rush things, take your time - it's worth it.
Especially if you "must" have an EC - that is the least of your concerns in the beginning, take your time to establish yourself and get a good foothold on the game

Raising cows for leather is probably the quickest. Crafting leather in the workshops nets you a bit more but it takes longer and you have to maintain the workshop so up to you if that's worth it to you.

Entering a bunch of competitions is also pretty quick (you get equus just for entering). Divine comps pay the most; and you get 3 free Divines from the Ascent tutorial so working them daily to build up a stock can be worth your while.

Other ways include selling excess items (I don't recommend just net-selling everything since some stuff will be useful in the future, but once you know more about what you will and won't use, you'll have a better idea) selling foals, and offering stallion covers.

One thing to note about dropping sales particularly; you can sell droppings but if you're not done your beginner objectives, you need around 500 in store to complete those, so don't sell them until you're done. After that, keep a stock of about 100 at a time. Daily objectives require between 60 -100 droppings to be sold for passes, so it's worth your while.

Raising cows for leather is certainly most profitable but not the quickest when you are new. They take 30 days.

. I suggest keep at least 2 meadows free for normal crops. It is currently autumn so you will need to use greenhouses to do it for now. Use manure to speed them up. Wheat and flax are traditionally the most profitable to sell in the store.

Adding the Parchment Paper bonus to you horses helps with equus.