I want to give it a try to creating a git repository from my UE4 project and uploading it to Github.com but I wonder what special considerations should I have to avoid posting sensitive information like my developer key/certificate, or avoid adding large binaries files, or temporary/intermediate files.
Things like your registration information for UE4 and whatnot aren't stored with the projects, they're stored elsewhere (with the application data for the UE launcher or toolchain, probably).
By far the easiest way to prep for posting your project on GitHub is to use the UE editor's ability to initialize source control access in the project. You do that by clicking the "Source Control" button in the toolbar:
In the resulting window, set the provider to Git (you'll want to make sure you have Git installed; if you do and have your Git configuration set up, it will populate everything for you). Make sure the checkbox to generate a
.gitignore is checked, and hit accept. It will do all the hard work for you.
If you aren't using the editor, if you're working directly with the engine from source or something, or you just want to do everything yourself, you'll want to set up a
.gitignore that basically excludes all the generated/intermediate file directories, like this (which is basically the one the editor would set up):
Binaries DerivedDataCache Intermediate Saved *.opensdf *.sdf *.sln *.suo *.xcodeproj *.xcworkspace
Beyond that, there isn't anything really special you need to do. You may want to consider something like git-annex to deal with large assets that you'll want to check in.