Welcome mate.
It's really a matter of trial and error and it is fairly difficult to advise which is the best route to take.
My first several games I ran out of money due to bad investments and plain bad luck in getting poor starting stables.
By all means don't feel oblige to follow these steps but they lead me to my first successful stable.
1. Start on easy, this gives you £200,000 and gives you the most horses you can start with. In my experience the Easy option does not make the races any easier to win (although no one has said it does or doesnt), to me it just gives you an easier start out in your career.
2. Don't be afraid to restart the game over and over until you get a good starting stable of horses (i.e) a few nice looking 2 year olds and a fairly nice rated horse, in my instance I was happy with a 80f which became a superstar for me earning 5.5 Million
3. Do not splash out on many Stable mod cons at the start. I only purchased the training stalls (advances starting speed). You are most likely to make a loss in your first year (happened to me in many of my games when i started out).
4. When I don't know a horse I never give any special instructions on the first run. I watch how the horse performs during the race, and also I read the jockey's feed back at the end. From these observations I choose the way the horse will run in future meets. You will start to get a feel for how a horse wants to run after experience.
If you have paticular issues or queries then by all means ask them, but it is hard to give a definative walkthrough for newer player as we may all have different styles and outlooks on how to manage a stable