Step 2: Setting Up
Trees work best in my opinion as anchor points for slacklines. Any sort of post in the ground will work, but I trust the root systems of most trees to support the tension and weight involved with slacklines more than a pole cemented into the ground. Find two medium size trees, big enough to hold a decent amount of weight, but small enough to easily wrap a line around. Palms, oaks, and pines generally work pretty well. It will be easier to keep your balance on shorter distances, so I'd look for two trees about 10-15 feet apart to start out. Technically, the trees can be as far apart as the slackline will reach, although there will be much more wobble in the line at longer distances.
Get some sort of tree protection before hanging the line. If you bought a slacklining kit, the included tree protection will probably be a couple of long strips of felt. Towels, cardboard, or carpet also work. The main purpose of the tree protection is to keep the tension of the slackline from damaging the tree bark. If you're slacklining in a public park, the rangers/police will appreciate this...
Experiment with different heights as well. With different lengths and tensions, the distance above the ground will vary. When you're learning, try and get the center of the line to stay about one foot off the ground when in use.