While WordPress itself is free, you will need a web hosting account and a domain name to use it on a live website. These are two separate costs and are best when purchased with the same web hosting provider.
You can get started by signing up for a shared hosting account. After that you can use a free WordPress themes with essential free plugins to get started. Hosting would cost you roughly around $100/year. The domain name is sometimes given free for the first year along with the hosting plan. Domain names cost around $15 per year.
As your website grows, you will need more features, tools, and services to better serve your users. Most plugins are free and usually offer a pro version with more options at a cost. Your goal will be to keep the costs down while offering the best experience for your users.