Hi!
When you start temping next cycle go to where you're seeing the calendar. Click on the day's date. Put in temp info, time taken, CM (it'll be AF at that point though), etc. Then save it and voila! FF makes the chart for you with that day set as CD 1.

When you have new temps every morning click on that day's date and save it just like before. It updates the chart. If you need to change or add something, click on that date on the calendar, or that date on the chart, and you'll be taken back to the data form.
Sometimes your temp will drop right before your period (a day, maybe two), and others it won't drop til that very day. If your period starts later in the day, actually, you might not see the drop until the next morning. It depends on the person and might also vary for you from cycle to cycle.
How big a drop is the same way ... everyone's different and every cycle is different. Normally you're looking for a drop below your coverline. When ovulation is detected FF will draw a big red "plus sign" on your chart with the lines intersecting on your O date. When AF is coming you'll usually see the temp drop right down to the horizontal line, or often even below it.
HTH!