In the 12 hour clock, use lower case for am and pm and don’t use full stops. The Style Manual (see last post) recommends that because am and pm are always preceded by a numeral, they can be treated like other symbols associated with numbers which are unpunctuated. Put a space between the numeral and the am or pm, so the time is easier to read.
By the way, am is an abbreviated form of the Latin ante meridiem (meaning before midday or noon) and pm for post meridiem meaning after noon.
A full stop should be used to separate the hours from the minutes (not a colon which is common in American English).
To minimise confusion between midnight and noon, use 12 midnight (instead of 12 am, representing the start of the 24 hour day) and 12 noon (instead of 12 pm).
For times on the hour such as 11 am, there is no need to show the zeros (11.00 am) as it doesn’t add any meaning and makes it harder to distinguish between 11 am and times past the hour such as 11.30 am.
These guidelines reflect the trend towards minimal punctuation. Punctuation is used to help communicate clearly. If it doesn’t help, then leave it out.