you just pick the right tag.
Best is you search first and look how the tags are written in other questions.
Usually the right tag immediately appears when typing the well known names:
Materials Management tag is MM (Materials Management) and I know that it appears immediately when you type M M blank or Materials Ma
The finance tags usually start with FIN
SD tags with Sales
You can find the list of primary tags here: http://sapassets.edgesuite.net/tools/SAP-Community-Tags.xlsx
Use Ctrl+F to search in the list, get familiar with the 3 to 5 tags that you need.
Click the yellow get started button on top right as the basics of posting are explained there.
Here you have actually chosen a wrong tag.
SAP Forum is an SAP event, like TechEd is a SAP Event, using this tag people can talk about this event. Your question is not related to this event. You would need to post such general question about posting in the community with the tag "Using SAP.com"