United Nations

Charter:

U.N. Charter art. 2, para.4.

Resolutions:

1. General Assembly
1a. Citation to the Official Record (GAOR)
G.A. Res. 832(IX), U.N. GAOR, 9^th Sess., Supp. No.21, U.N. Doc. A/2890, at 19.

1b. Citation to electronic version found in the Official Document System (ODS)
G.A. Res. 62/228, ¶ 35, U.N. Doc. A/RES/62/228 (Dec. 22, 2007)

Before 1976 (31^st  session), the resolution number is indicated as a Roman numeral in parentheses after the resolution number. After 1976, it is cited in Arabic numbers.

2. Security Council
S.C.Res.1325, ¶ 8, U.N.Doc.S/RES/1325 (Oct. 31, 2000)

Source: 
The Bluebook: a Uniform System of Citations (password)
(Cambridge, Mass.: Harvard Law Review Association, 1991-)
340.11(035) HEIA 11825

Charter:

Article 13(1)(a)
In footnotes, « Article » may be abbreviated as « Art. »

Resolutions:

1. General Assembly
1a. Citation to the Official Record (GAOR)
G.A. Res. 832(IX), U.N. GAOR, 9^th Sess., Supp. No.21, U.N. Doc. A/2890, at 19.

1b. Citation to the electronic version found in the Official Document System (ODS)
G.A. Res. 62/228, ¶ 35, U.N. Doc. A/RES/62/228 (Dec. 22, 2007)

Before 1976 (31^st  session), the resolution number is indicated as a Roman numeral in parentheses after the resolution number. After 1976, it is cited in Arabic numbers.

2. Security Council
S.C.Res.1325, ¶ 8, U.N.Doc.S/RES/1325 (Oct. 31, 2000)

Source: European Journal of International Law

Generally speaking, cite UN documents in the following order: author, 'title' (date) document number.

UNSC Res 1373 (28 September 2001) UN Doc S/RES/1373

UNGA 'Report of the Special Committee on Principles of International Law Concerning Friendly Relations and Cooperation Among States' UN GAOR 25th Session Supp N° 18 UN Doc A/8018 (1970)

Source: Oscola 2006: Citing International Law Sources Section (pdf file)