Jeff, I'm a new TH'er like yourself. I'm planning to start with public property to familarize myself with my equipment & gain alittle confidence before moving-on to bigger & better things.
One of my prior hobbies required speaking to land-owners inorder to gain permission to use their land, and I feel some of the principals would still apply.
Be aware of your personal apperance, as well as that of your vehicle.
Be curtious as to what time you ring the doorbell. Avoid suppertimes, etc. Also know when farmers are busy (if in a rural setting), "bugging" anyone at times like these will not be a positive thing.
Be sure they know you're respectful of their property. I used to carry a bag & pick-up any trash I found.
Close gates behind you if they were closed when you found them.
Start with folks you know, alot of times neighbors may see & allow you on their property as well.
Print-up a card with your name & contact info. You can always leave it if you get a "no" in case they change their mind.
There's a post here on the "Legal" forum with a good agreement on it.
There's a few, it's past my bedtime, so mental capacity is rapidly diminishing
Welcome & Good Luck!
Smitty