Welcome to the forum! I'm new myself
By the 'shall' I presume you're about to buy a horse. If you're not experienced in horses and horse ownership (which I presume from your question, I apologise if I'm wrong), you will need an experienced horseperson or instructor to help you. It's also a very good idea to take lessons previously so that you can have an idea as to what to expect and will have a little experience.
If you decide that taking lessons are fun and interesting (not just after three lessons, but for months), perhaps look at leasing a horse. This isn't as big a commitment as full-time ownership but gives a good idea as to what ownership encompasses.
Then, once you have a little more experience and you're still interested, you can look at buying a horse. Remember horses are a big commitment - you need to visit them several times a week minimum and spend many, many hours with them, for a lot of years. Horses aren't just a pet, but a lifestyle.
Onto your question about feeding:
Where I take lessons, the horses are fed a biscuit of hay each twice a day with a scoop of pellets each in the evening feed. They have unlimited access to water throughout the day.
I don't own a horse, but that is what I would do in any case, and how I would feed. Owners please correct me if I'm wrong!
There are different forums on this board for different topics to help members find threads they can contribute to. It can be hard to find the right forum to put threads in, but the Horse Nutrition section would probably attract more replies from members more experienced in this field than I am