There are lots and lots of articles and videos that tell you a lot of details about what you should pick or what you should write, you can study a lot before writing the first letter in the title of your first gig. You should also study your competition. Fiverr is a jungle, with a lot of people competing on the same services, so you should find your niche... and be noisy.
Choose the categories that fit you. It does not matter if you work with Wordpress or Joomla, it's not so important that you are using Shopify, Woocommerce or CakePhp for the online stores you are developing, what's important is that you love working with that technologies that and you are great with them. What you can provide for your clients is making the difference between you and all the others. It's not important if you are building entire websites or you are just fixing broken things on existing sites, post your gigs with the most detailed description you can get. Adding new small features on some Wordpress or Joomla websites is great if it's building your reputation. Remember to study your competition, use the best part of what they write in their gigs, be unique and get people's attention with your services.And don't forget to post all your gigs on social media. Twitter, Facebook, YouTube, LinkedIn, whatever works to bring you clients. Be noisy, tell the people what you are doing and how good results you are getting.