Easy as it sounds, there are numerous different approaches. I like to keep my websites non-www and SSL secured. Just copy-paste the code below to your .htaccess file and you should be ready to go. It is universal.
Keep in mind, you have to adjust the code if previous modifications to your .htaccess file were made.
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# Rewrite HTTP to HTTPS
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*) https://%{SERVER_NAME}/$1 [R,L]
</IfModule>
# END WordPress