How and why is the ‘president’ of the UN chosen ??

Question

Are there specific criteria's to be met ??

Answer 1

If you're refering to the secretary general for the united nations this is how they are selected. A person is first nominated by the security council. So basically, the person is selected by the 5 permanent members with the veto power. The secretary general is actual appointed by the general assembly after the nomination by the security council. There are many regulations about this as well. For example, the person cannot be a citizen of any of the 5 permanent members of the security council.  

Answer 2

Dude, don't say "president" of the UN.  Say "Secretary General'

Article 97 of the United Nations Charter establishes the procedure for the selection of the Secretary General.  The final word is of the General Assembly, in which all United Nations member States have a vote.  That decision must be approved by a majority of two thirds of the members present and voting, in virtue of article 18 of the Charter.  But before the Assembly decides, the Security Council must recommend.  There, the indication of the Secretary General must be approved by the affirmative votes of 9 out of the 15 members, and out of these 15, the Permanent Five (P5 - United States, Russia, China, France and the UK) cannot vote "no" (although UN practice consolidated the understanding that they may in fact abstain).  This is provided for in article 27 of the Charter.  

Now, as to why, the Secretary General is chosen to fulfill the position of the person who is the head of the UN personnel, that is, its administrative staff, and also the representative of the UN as an international organization for the purposes of its international relations with other entities (States, orther international organizations etc.)

As to the specific criteria, the Charter doesn't provide for any, but practice of the UN has been in the sense that an individual yielding from a "neutral" country (in relation to the P5) should be indicated.  The Charter does not provide for age, educational or experience requirements -- that is all left for decision on a political basis.