I measure the 1/4 or 1/2 inch headspace from the lip ( the very top egge) of the mason jar. I mostly eyeball it but for those items I am entering in the county fair, I measure those...because the judges do.
When I take the jars out of the hot water, I empty the water out. The jar is very hot so by the time you get it to where you will be filling it, it has already air dried. If you dry them with a towel or paper towel, you have just unsterilized your jars. Same for the seals.
I do not remove the bands till the jars have cooled after processing...usually overnight. AKF explains well, how to test for a good seal.
You will need to get basic processing tools...tongs for the jars, canning funnel (it has a wide mouth and fits mason jars. And invest in Ball's Blue Book of the one Kerr puts out. Ball has illustrated, step by step instructions for basic canning tomatoes and peaches. It also has safe handling guides. I have canned for years, yet I always refer to the guides in Balls. Also always and only use mason jars made specifically for home canning...Balls, Kerrs, Golden Harvest. It is a big initial investment, but you can use the jars over and over...some of mine I have used for 25 years.
Good luck. Caninng is fun....home made jams and jellies beat store bought - even expensive "gourment" jams - hands down. Nothing better in February than to pop open a jar of "summer goodness" from your own garden.