The first thing I've EVER shipped was my GE package

. I used UPS which is like 20-30 miles away.
I have a few things I want to sell on here (very few dog things & quite a bit of "garage" type things). What would be the best way to do it? Should I just use USPS? Should I go ahead and pre-buy my packaging at Wal-mart or wherever? Do I just wait and ship everything at the same time or ship them whenever someone wants them? Best way to tell how much it's going to cost to ship the items?
I just want to do it the best possible way to please everyone therefore I'm asking before I proceed.
Thanks for bearing with my pain in the butt self.