I am a beginner in this field and I do not know where to start. Do you know a good course to start with?
Welcome to NamePros! In case you have not found it, this section has a good collection of NamePros articles specifically aimed at those starting in domains. They first item they mention in a three part series I did on just the basics.
https://www.namepros.com/threads/popular-domain-beginner-resources.844937/
how long does it take to sell the first domaine and what is the price I sell it for? Does each sale exceed $200?
It can take anywhere from days to a decade or more. Literally examples of both. We use something called STR, or sell-through rate. It gives a rate of sales. For example, if you had 100 domain names and after one year had sold 1 of those names, your STR is 1%. That is about the industry-wide average for .com domain names, but individual STR can be much less (for those who hastily registered low quality names) than this to somewhat more (for those with skilled buys and doing other things to increase chances of sale).
Re price, most retail sales are indeed at prices above that ($1000 to $3000 is where many fall), but there are sales from just tens of dollars to literally millions of dollars. There is a solid argument to do some selling to other investors as you get started, as this will give you feedback on what is considered worthwhile. But eventually, you will only make enough to be worth the effort, most would sell, by concentrating on retail sales at higher prices.
You should not invest money that you need for everyday needs, or safe savings, since there is no guarantee a domain name will ever sell. This is definitely not fast, easy or assured. It is true that many do well, but often only after years of struggling to master the fine points, and many others drop out never having achieved profitability.
Sorry this has been so long. I wish you the best, and you will find NamePros a valuable source of information and opinions on almost every topic related to domains.
Bob