How do I group tests into tests suites in Test Complete?

I'm assuming all your tests are in a single project?  

There are multiple ways of doing what you want.  The EASIEST is to go is to go to View | Organize tests.  This will take you to the "Test Items" panel of your project.  For every test you want to run, add a test item here to point to your test and organize them in the desired order.  When you run your project then, that will run the tests.

Now, you can get more complex by writing code, keyword tests, etc., that,themselves, run the tests.  There are any number of strategies where you can build a coded framework to execute the desired set of tests.  I would strongly suggest aiming in that direction in some day because it will allow you a much greater flexibility in constructing report output, error/exception handling, conditional test runs, etc.