Once you are ready to enter a race, you will have to select a jockey. Each region has certain jockeys that are available, which will appear on a dropdown list on the entry page.
There's not much difference in what jockey you select initially. They have their own stats, but these are low and will have a minimal impact on your horse's performance. However, horses develop a relationship with their jockeys, and race better when running with someone they are familiar with. Therefore, once you select a certain jockey, it is best to try to keep that same jockey on your horses every race.
Once you save up some money, you can can contract your favorite Jockey. Doing so will make it so only you can use that jockey, AND you will be able to enter with them in any region you wish to run in. The biggest benefit of having a contracted jockey is a bonus score in race, which adds on top of the familiarity bonus you are hopefully already getting.
Until you can afford to contract a jockey, the two things that can help you get your favorite jockey every week are (1) entering as soon as possible and (2) selecting a jockey you don't see a lot of other people using. Even if it's not the best jockey, it is better to have one you can use consistently than to flop back and forth every week.
You can create custom jockeys from the normal jockey management page. When creating a jockey, you are able to choose their first and last name (please be realistic) as well as their trait. Their contracts will be good for 1 year and will be renewable when that year is up. You will never lose them, even if you choose not to renew them right away. You'll also have the option of changing their trait at the time of renewal. Custom jockeys can also race in both flat and SC races.
The jockey page has a list of the available traits with descriptions for each. In a nutshell, traits give custom jockeys an edge in various situations. Each jockey can only have one trait, so no single jockey is going to have an edge 100% of the time. You can also only hire up to 5 custom jockeys, so you'll have to pick and choose which types will work best for your stable. In situations where the trait does not apply the jockeys will perform just like any other jockey.
Are custom jockey worth the price? Yes! You will never be able to know the talent of a normal jockey so you could have a bad or average jockey that will never be as good as a custom jockey because created jockeys will always be the best. If you want your horses to perform good in G1 races having a custom jockey is really at your advantage!