It is fairly simple - Contact the person selling the item through a Private Message (PM). You can do this by clicking the little chat bubble icon below their name on the forum post. Let them know your are interested in the item. Give them your zip code/postal code/country so they can calculate the cost of shipping. Then the seller will let you know your total price (cost of item plus cost of shipping). You then send the payment to the seller via Paypal (sometimes by money order, but most people use paypal). The seller will then ship the item to you.
A few things to be aware of:
ALWAYS ask about insurance.
Tracking is a good idea, too, so you will know where the parcel is.
If the item does not arrive, be aware you can open a paypal claim, but don't do it too soon (usually allow 30 days, but claims do have a time limit of 45 days).
Most sellers are friendly and are easy to stay in touch with. Contact your seller if you have any problems. If the item arrives broken, not as described, is taking a long time, etc.
If you need help, there is a Traders and Classifieds Support forum, where you can post to ask for help.