Write the time with the hour, then a colon, and then the minutes. For example, you might ask to meet with someone at twelve fifteen by writing 12:15.
Express whether you wish to meet in the morning or evening by using the terms "a.m." or "p.m." Thus a meeting at lunch time would be at 12:15 p.m.; a.m stands for ante meridiem or "before noon," while p.m. stands for "post meridiem" or "after noon."
Writing a description of the clock's hands hitting 12 is slightly different. Since a.m. and p.m. describe before and after noon, there is no description for 12 o'clock. In this case, simply express it as noon or midnight. No numbers are required.