I know two methods:
Important : Do not force HTTPS if your site does not have SSL certificate.
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
Or
RewriteEngine On
RewriteCond %{SERVER_PORT} 80 /*porta que utiliza*/
RewriteRule ^(.*)$ https://site.com.br/$1 [R,L]
/*Linha 2: Condiciona que todo acesso vindo da porta 80 será afetado pela regra;*/
/*Linha 3: Definição da regra, neste caso, sempre utilizar o https:// mesmo quando acessado por http.*/
UPDATE1
For only 1 domain:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^seudominio\.com [NC] /* aqui é como um if */
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI}
or if you prefer something clearer, but it does the same thing:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www\.)?seudominio.com$
RewriteCond %{HTTPS} off
RewriteRule ^ https://www.domain.com%{REQUEST_URI} [NC,L,R]
If it is useful, do not forget to evaluate, this helps a lot, abs