A test case is just a work item. You can create a new one from Team Explorer in Visual Studio. You can open Excel and connect to the server from the Team add-in, and create one from there. In MTM, you can also go to Test Case Manager and create test cases.
Those are all fine, but if you are authoring test cases in the context of a test plan or requirement, there is a better way. When you are in Test Plan Contents (the suite manager of MTM) you can add existing test cases to a suite, and you can also create test cases from there as well. You get the same experience of the work item form, but when you are done, the item is also added to the suite.
Note, this works only for static and requirement-based suites.
In the static suite case, the test case is added to the selected suite and all default configs specified by that suite are added to the new test case.
In the requirement-based suite, the test case is linked to the requirement, which adds it to the suite, and all the default configs specified by that suite are added to the new test case.
If you didn’t author the test case from Test Plan Contents, then you’d have to manually go add the test to the suite of your choice after the fact. I said this was better, because it helps save you time by accomplishing both tasks at once (create, add to suite).
Engineering Lead, MTM