If you don't know the exact weight of the item you can't give the buyers an accurate price quote. Even an ounce can make a difference. Items that weigh one pound and one ounce must go at the two pound rate. For that you need an accurate scale or you need to box up your item and take it to the post office to be weighed.

Once you know the real weight you can enter it into the shipping section of the listing form. You check which method of shipping you will be using and when completed it will show each customer the price to their location. (As stated in the other answer sometimes you also need the size of the package).

When you print out the shipping label when a customer buys your item you pay the total shipping then. It's the price of the label. Tape or glue it onto your box and take it to the post office or arrange a free pickup if it's First Class or Priority Mail.

You need more than a postage stamp on a package. Look at their website and see what boxes they offer. You must weigh the item and use the Ebay calculator or put a flat rate for everywhere on your shipping section before you list. You can use your own box if it is cheaper than Priority Flat Rate or Priority. Some items can be mailed First class, some Parcel post, some Media rate, slower but very cheap. There is no "Ebay" postage. You can print a label from either the post office or Ebay which deducts cost from your Paypal.

you need to know the weight and sometimes there are size limitations - depends on what you're shipping

I know about flat rate. But sometimes that can be pricy.

I was told you can ship with just a regular cardboard box, but do you just put the ebay postage stamp on it and ship it, or do you need stamps on it too?