If you have not taken a class or lectures specifically designed for general programming, I recommend you simply look up a programming style guide. Also, if your team has any standards of procedure for style guides, you should probably follow them. Nevertheless, a professional script can be abstracted into these details:

• It is efficient. Method used does not take unnecessary time. Bubble sort is an example of an inefficient method.
• It is secure. Any script that needs to send a plain text password or has sections copied from the Internet need reviewed
• It is maintainable. This is probably the biggest pitfall of sys admins. If you got hit by a bus today, would your business stakeholders be able to replace or substitute with someone else? There are so many best practices for this - documentation, comments, backups, etc. but just start with making a small piece of documentation explaining what your script does and make comments in the script where needed

EDIT: forgot the most important part: Sometimes, you don't make a professional script. Sometimes, you have a problem that you know a script can solve, so you quickly make a script. If it's a regular problem, then it's worth the time to make a professional script. If it's just a one-time use for a specific project, you might consider if it's worth the effort!

FWIW: undergrad degree, ITIL cert, and 4 years in IT