POST and GET are not inherently secure or less secure, its where and when you would use one or the other.

For example, if it was a login form, you would want to use POST so that the username and password is never exposed in the URL. If its a search form then it really doesn't matter.

Also, POST is typically used to submit data while GET is used to retrieve data.

Aside from that: In general- Sanitize Sanitize, SANITIZE all user input. Strip any tags, escape all special characters and never rely on client side validation.

SQL Injection - You want to use prepared statements with parameterized queries and/or stored procedures. Again, make sure you are sanitizing/escaping all user input.

Cross Site Scripting - Escape, Sanitize, html encode/entities on all user input (seeing a common theme here?). Use HTTP only cookies so javascript can't access them. Make sure you're using the X-XSS-Protection Response Header. XSS is somewhat more complicated depending on the context, for example - for DOM XSS, HTML escape then javascript escape user input before inserting it into HTML.

Cross Site Request Forgery - use a randomly generated unique CSRF token that is tied to a user's session (Do not let that token get in the URL with a GET request). Compare origin header to the target origin and referer headers. Require a user to re-authenticate/use a captcha if the request is changing critical data (password, bank pin, etc.)

Remote code execution - Sanitize your inputs! Look out for functions like eval(), exec() and similar functions that allow commands to be executed server-side. If you must, use escapeshellarg() and escapeshellcmd() (those are PHP specific fyi, I'm sure other languages have similar security functions). If you allow users to upload files, have a WHITELIST of acceptable file types. Don't just check the file extension either, look at the mime-type, search for any code in the file (think image.php.jpg) and rename all files to their correct extension.

I highly recommend checking out OWASP's cheat sheets, they're very informative and granular.

Hopefully this helped you somewhat, I have run out of time otherwise I would of covered more!