One simple way to add meta descriptions is to use the {summary} tag in an article template’s header.
Wrap it within the exp:channel:entries tag and limit it to one entry.
Taking it a step further, you can add a field for the entry that is called Meta Description and call that one instead of Summary if you want something different than the summary.
That way the description is dynamically generated for each entry and you also can use custom fields.
For the section fronts, skip the channel:entries tag and just manually write the description.
Likewise, you do the same thing with titles.