Creating issues
Open the new-issue form at /:owner/:repo/issues/new.
Required fields
Only a title. Everything else can be added later, because the cost of an unfiled thought is higher than the cost of an under-specified issue.
Setting labels and a milestone at creation
Labels and a milestone can be attached on the form rather than in a second pass. Both must already exist in the repository — creating a label from the issue form would let a typo become a permanent label.
Numbering
The number is assigned on save and is sequential within the repository. It is not global, so two repositories both have an issue 1.