From the main view of your project, select the Issues menu option on the left-hand side. Although the name typically refers to problems, it's here that you'll be doing a lot of the project management work, including setting up milestones, writing up feature requests, and divvying up work to different contributors. You should be faced with a screen that looks something like this:
Issues
Creating an issue
Click the New Issue button and you'll arrive at the Create an issue screen. This has a variety of fields that you can fill in to report on a problem or desired change in the GitLab project:
Let's fill out an issue. I think we should probably be using Composer (https://getcomposer.org/) to manage this PHP...