Well I Use apache not nginx so I can’t help you there but at the certbot page you can enter your webserver and Linux variant to get some instructions. If you need more basic instructions on setting up nginx for https just googling “nginx https setup” gets lots of results.
The free letsencrypt certs are just as good as the paid ones except that they expire after three months but you setup certbot with a cron job to automatically renew the certs before they expire. Instructions at the certbot site. I use certbot standalone mode rather than an apache plugin for various reasons and you could also do this with nginx - just use --pre-hook and --post-hook to stop and start nginx to allow certbot to bind to port 80 temporarily to authenticate.
Googling letsencrypt and or certbot will get you much more information.