I would say that knowing the law is key. It doesn’t cost much to hire somebody to manage the finances, but getting help with keeping track of all the laws, rules, and regulations does cost, so it’s better to do it yourself. If you can also do the numbers yourself then all the better, but if you must choose only one then learn the law.
Aside from that, I am in total agreement with @Jaxk, especially about making sure you have a stash set aside to handle unexpected expenses and not trusting your tenants to pay on time (which is probably one of the most common expenses). Even if you do everything right, there is no guarantee that the people who are supposed to pay you will, and you can’t reap big rewards unless you are willing to risk being ham-strung by the actions of others.
It can be rewarding, but it can also be a royal pain in the balls.