While WordPress itself is free, you will need a web hosting account and a domain name to use it on a live website. These will cost you money.
You can get started by signing up for a shared hosting account. After that you can use a free WordPress theme with essential free plugins to get started. This would cost you roughly around $100/year.
As your website grows, you will need more features, tools, and services to better serve your users. Some of these services and tools will cost additional money. Your goal will be to keep the costs down while offering the best experience for your users.