Before you will start to make the transition from HTTP to HTTPS you have to be sure that you have a SSL certificate installed.
Also you will have to check that your website is loading on a secure connection. You can do that by accessing https://yourdomain.tld.
Within this transition proccess, you might be in the following situations:
Part 1:
Case 1. Green Lock:
This means that your website working fine on a secure connection. and you can skip to part 2.
Case 2. Red lock or exclamation mark:
If you will encounter an red lock or an exclamation mark (like in the picture bellow) this means that your content is served trough an insecure connection (http). 
To see what insecure content is served, you will have to load your webiste over HTTPS. After that, you will have to press the F12 key. Select the console and refresh the page. An example of insecure content can be found bellow:
Mixed Content: The page at 'https://domain.tld/' was loaded over HTTPS, but requested an insecure stylesheet 'http://domain.tld/css/screen.css'.
Bellow you will find the steps on how to change the content to be served on a secured connection.
Part 2:
The transition will differ from platform to platform. Bellow, we will try to cover as many platfroms as possible.
a. Custom website with no CMS platform installeld:
Case 1:
The redirect will be made trough a .htaccess file, by adding the following lines:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
Case 2:
If you don't have the green lock and your content is served trough an insecure connection, you can correct this by rewriting the links inside your website. An example can be found bellow:
old: <link rel="stylesheet" href="http://domain.tld/css/screen.css" type="text/css" />
new: <link rel="stylesheet" href="//domain.tld/css/screen.css" type="text/css" />
b. Wordpress website:
After you login into your wordpress administration panel, you will have to access Settings -> General and you will have to modify the link like in the picture bellow:
*domeniu.ro is just an example and should be replaced with your domain.
If after this change the website does not redirect automatically https. You can install the following plugin create de redirection:
https://ro.wordpress.org/plugins/really-simple-ssl/
If the redirection is done but the website is still not fully secure, then you should install this plugin:
https://ro.wordpress.org/plugins/ssl-insecure-content-fixer/
c. Joomla website:
You can make the transition to HTTPS by installing this plugin: http://extensions.joomla.org/extensions/extension/site-management/url-redirection/yireo-ssl-redirection
d. Prestashop website:
After you login intoo Prestashop administration panel you will have to go to Preferences -> General -> Click on Please click here to use HTTPS protocol before enabling SSL.
If you receive an error like (Internal Server Error 500, 404 not found, 102 (net::ERR_CONNECTION_REFUSED)) please contact us.
If the page is loading fine, you can select Yes from Enable SSL and you can click on Save.
e. Magento Website:
Ater you login into Magento Administration panel, you will have to go to System -> Configuration. Here you will have to click on Web under General tab. In the new window you will have to select Yes for these two options:
Use Secure URLs in Frontend
Use Secure URLs in Admin.
If you encounter any issues with the transition to HTTPS do not hesitate to contact us.
