@whu . You have received some excellent insight from a bunch of people who are really very good at what they do. However, I sense that you are a complete beginner and therefore I am going to tackle the issue in a different way. It is then up to you which way you choose to go.
Working out your own ratings is a very difficult science indeed and unless you are either very good or very lucky, with your current level of expertise you are likely to get it wrong and lose a lot of money. If you follow the JIB method or any other method really, you need to understand a bit of the logic and all of the terminology behind it. I suggest for starters you go slowly and educate yourself by subscribing to Timeform. Not cheap but they provide full and detailed explanations of what they do and why they do it. Once you have mastered that and you know what ratings are all about, then I suggest you move on to the excellent sets of different ratings on this site. When you feel that you have mastered the logic behind those, then and only then try working out your own ratings.
Sadly, there is no magic bullet, and it may take you years to get to the point where you start making regular profits.
Also never forget that ratings are simply an expression of opinion, and so different sets of ratings will produce different suggestions as to the winner of a race. You will have to exercise judgement, and that only comes with experience.
I have no connection to Timeform, except as a long term subscriber , and I only suggest them because they do produce good explanations of their methods. Whatever you choose to do, you will learn much from the wealth of wisdom on this site. First of all treat racing as a hobby, and if you make money at it, so much the better!