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Besides, is 12am a midnight?

Another convention sometimes used is that, since 12 noon is by definition neither ante meridiem (before noon) nor post meridiem (after noon), then 12am refers to midnight at the start of the specified day (00:00) and 12pm to midnight at the end of that day (24:00).

Likewise, why is noon 12 pm? The abbreviation am stands for ante-meridiem (before the Sun has crossed the meridian line) and pm stands for post-meridiem (after the Sun has crossed the meridian line). At 12 noon, the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither 'ante' (am) nor 'post' (pm).

Keeping this in view, is 12 o'clock AM or PM?

… and 12 noon is between AM and PM. But, by convention, people have decided to make it 12 AM. By convention, 12 am refers to midnight at the start of the specified day (00:00 on the 24 hour clock) and 12 pm to midnight at the end of that day (24:00 on the 24 hour clock).

How can I remember 12 AM and PM?

See the following from the Wikipedia entry on the matter of the 12 Hour Clock. “It is not always clear what times "12:00 a.m." and "12:00 p.m." denote. From the Latin words meridies (midday), ante (before) and post (after), the term ante meridiem (a.m.) means before midday and post meridiem (p.m.) means after midday.