Go To
http://go.sap.com/community.html
Click on community --> Browse the community
Type in key word for the community you are looking for , I am taking an example of SAP SD Sales and Distribution , I will use “Sales” as key word
Type Sales and hit enter
I found 3 results , in left I can select/sort it by product or software product function
Select the “ SD (Sales & Distribution)
You can filter between blogs or questions by selecting radio button
If you regularly use this community you can click on follow and it will be send you update on new content in this topic area.
To see list of all the Tag's use below link
http://sapassets.edgesuite.net/tools/SAP-Community-Tags.xlsx
you can open the excel sheet and search for tag you are interested in and click on follow, it will setup notification for your profile for that specific tag.
Link to search Blogs & documents in SCN
https://archive.sap.com/