Its not easy selling online and my first bit of advice would be don’t expect too much too quickly! If you do take the plunge and decide to sell here then join in the forums there are really helpful people here and there is a thread where there is lots of advice.
Good photos are very important and an accurate description.
As far as postage is concerned just ask your post office for a booklet on prices and decide whether you want to send things standard delivery (insured up to £20) or signed for (insured £50+). Always get proof of posting and be aware that paypal seller protection is only available if you use a tracked service (I think). Don’t forget to include your packaging costs and the cost of getting to the PO into the postage cost. Also make sure to factor into the price of your item your listing fee, paypal and selling fees.
Hope that helps - good luck