Hi Carol,
Here's a link to Ebay's 'getting started' link:
http://pages.ebay.com/sellerinformation/starting.html
You have to set up a seller's account, have a bank account added, paypal account....
This link should let you know everything you need. You can also click on 'community' link, top right, for more answers on just about everything.
It's also helpful to have an electronic scale (I bought mine on Ebay), to weigh items to determine accurate shipping costs, which usually the customer pays. You can get shipping costs from usps.com, and I usually put in a zip code which is quite far away, to cover whoever. I usually ship books, etc, if over 13 ounces, by media mail, as it's the cheapest form, although takes a bit longer.
I would also advise including several pictures, listing anything which might be wrong with the item - torn page, page marked on, etc, - will save you from getting negative feedback, and also you'd want to know the same if it were you buying it. If you smoke, or have pets, let people know in case such things bother them.... Also always put books, etc, in a ziplock baggie, (I also buy the padded mailing envelopes) to ensure no water damage while shipping.....
And how many per sale is up to you. Some sell individual magazines, others sell several together....
Good luck, and have FUN with it!! I've sold a wide assortment of things, and so far, so good. I have feedback of 180, 100% positive. :)