Are you trying to create a featureclass and edit it?
If so you have to create the empty featureclass first, then you can edit it.
That process is covered in the 2nd link I posted.
If you are trying to edit an existing featureclass, then you have to ensure that you have permissions to edit it. It should be stored locally on your machine and under your control. If it is located elsewhere and is 'owned' by someone else, then you might be edit a locally stored copy, but rarely will you be allowed to edit a copy.
Also, to start editing something, it has to be selected in the table of contents ... the procedure for getting ready to create features and editing existing features is pretty well laid out in the help files, so you are going to have to be specific about what steps you followed and at which point do you not get past.
