I just did the same thing as part 1 of your question yesterday. As a prerequisite, you will need a container registry to put things into, such as a free account on Docker Hub.
Start on the Code Engine console.
Choose Start with Source Code, and paste in your on Gitlab URL. (The default is a sample repo which may be useful, especially https://github.com/IBM/hello.
On the next page, you can accept most of the defaults but you will need to create a project. That's free, but it needs a credit card so you can use a Pay As You Go account.
You'll also need to Specify Build Details
Here you tell it about your source repo, and where your Dockerfile lives. If you don't have a Dockerfile, you can probably find a sample one for your chosen runtime (C#? Node.js? Java?), or you can try using the Cloud Native buildpack. The build pack will try and work out how to run your code by inspecting what files you have.
Finally, on the last page of the build details, you will need to tell it where your container registry lives. This is a repository used to store the built images. If you set up a personal account on docker hub, you can just enter the credentials.
Once you've done that, you can choose Done in the sidebar:
and then Create
You should get a page which shows your image is building
and then once the build is done, in the top right you'll get a link to take you to your app's web page:
If you get stuck, there's a good set of documentation.
