How much power do you get from being a part of the United Nations? What is a country able to do? What can it not do?

A country does not gain any real power by becoming a member of the United Nations.  There is not UN Army that the country can call upon.  There are no special powers that UN countries have and others do not.  For example, Taiwan is not part of the UN (because China will not allow it).  This does not cause it to lose the ability to trade with other countries, to have its international boundaries respected, or anything else.  Membership in the UN simply does not bring with it any real powers.

If UN membership carries no power, why are essentially all of the countries in the world part of the organization?  First, it means that your country is a recognized part of the international community.  Kosovo, for example, would like to be in the UN because that would mean that the international community recognizes it as an independent nation.  However, this is a matter of prestige, not one of power. 

Second, being in the UN does give countries access to some money through UN programs that help countries develop.  A poor country might like to be in the UN because the UN will have programs that can give that country money for education, for health infrastructure, and the like. However, even this is not a really important reason to join the UN as the UN does operate in Kosovo even though Kosovo is not a member state.

Finally, being in the UN gives countries an easy way to talk to the governments of other countries.  One of the main purposes of the UN is to allow countries to interact easily so that they can resolve conflicts.  The idea is that countries that can talk to one another frequently will not end up going to war.

Because of the nature of the UN, there is no real power that comes with membership.  Countries belong to the UN more because there are a few benefits and because that is what is expected of all countries.  They do not join to gain any tangible power.