In sentence 1, "go" is meant to be hyphenated with "to" as "go-to." If something is "go-to," it means that you look to that something for whatever reason. For example, "Timmy is a go-to scorer for his basketball team" means that the basketball team looks to Jimmy for scoring. So in your sentence, "go-to communication tool for activists ..." means that Activists look to use Whatsapp as a means of communication. Sentence 2 says "North Korea will be high on the agenda ..." An agenda is a list of things to accomplish, so here's it's simply stating that North Korea will be of top priority in the agenda.

1)GO is actually not standing/used alone in the sentence; it is used with TO, so the word is "GO-TO." Go-to is an adjective that means "Effective" or "Reliable." 2) The use of 'High' in the second sentence is to show the position of "North Korea" in the ranking of issues that would be discussed at the UN general assembly. It basically means "North Korea" would be discussed more than others.