Scripts? PHP, javascript or something else. First have a goal or problem to solve. Do a Google search for a solution. Study and understand the solution. Implement it for yourself. Verify that it works.
Pick a language you feel confident in(PHP, JS, ASP, etc) Find a problem, maybe something that you need personally, or that you see a lot of people on forums need, I would like an easy list management software i can install on my website for instance Figure out the components that you would need for your script(i need an api that does this, a form that submits this, a database with these tables) You now have a grasp of what you need the script to do, you can now start making a wireframe for the design(or hire a freelancer) Start programming your script, set milestones to keep you motivated(today i want to finish this function/class) Start optimizing code, minimize database queries, seperate HTML, JS and PHP into their own folders, this makes the project easier to maintain if you want to upgrade it in the future. If you intend to sell the script, make a nice logo and some marketing materials(Box Cover art etc) Setup a sales page with a feature list, screenshots and technical information. Done