This isn't a black belt rocket scientist VSTS tip by any means but it's an important one to master if you ever want to log a bug. To create a new work item (AKA "bug"), right click on the Work Items folder in the Team Explorer window and in the resultant context menu you will see something like "1 Add Bug..." - choose that and a form will come in that you can fill in with the bug information.
I say "something like" because the actual name of the work item depends on the customizable work item type defined during or after project creation. They can be called defects, issues, anomalies, or whatever else you want to replace the word "bug" - or all of the above. It is possible to have multiple work item types so there might be a couple different options in that context menu.