1) Click Help & Training in the upper right corner of your SFDC instance
2) Click the "Log A Case" button under the Contact Support box on the bottom right of the screen
3) Click on the Topic box that you have a question about
4) If it's Billing Help, it will bring you to the Case form. All others will give you either a set of questions or sub-topics. Keep clicking through until you get to the questions.
5) If you don't see a question related to your issue, click any question and you will see a "Your Selection" screen with Recommended Content and Trending Content.
6) Below those 2 sections, the 1st green box has Ask The Community and Log a New Case
7) Click Log a New Case and then update the "I need assistance with" and "Subject" lines as appropriate.
If you need to speak to someone, the numbers for support are here: