BBT help?

I'm new... my cycle is a little jumpy so I am trying to track my bbt. I wake up at different times each day for work and have been using a regular thermometer. I have only been charting for about a week but could someone please explain to me or tell me where I can find a good explanation of what the chart actually means? I have also seen that there is a BBT thermometer should I be using one of those instead? Thank you :)