Give in full at the first use, unless the abbreviation or acronym is so familiar that this is clearly unnecessary (e.g., BBC, DNA, USA)
Most acronyms should be written without full stops:
BBC, TUC, BLitt, USA, PhD
Where the abbreviation is partially or wholly lower case, no full stop is needed if the final letter of the abbreviation is also the final letter of the full word:
Mr Dr Ltd
Where the abbreviation is all lower case, the full stop is increasingly omitted:
mph plc am pm oz lb
NOTE these exceptions:
e.g. i.e. p. pp. ch. (BUT chs) no. (BUT nos)
Use a full stop in these cases:
ibid. et al. (NOT et. al. Also note that et al. is NOT italicised)
Do not use in initials, which should be spaced:
Dr A J Smith; S M Barney; Crankers, O J; Thomas M Jones
Do not use a full stop at the end of picture captions or at the end of headings.
When an abbreviation can be pronounced (NATO, UNESCO), the definite article is not necessary. Where it can’t be pronounced (the BBC, the USA), include the definite article.
From the Writing and Style Guide University of Oxford, England