The sooner you get it in the ground the better (unless your ground is currently frozen or soon will be).
The more sun it gets the better (IMHO). At the moment you have a small tree, but when selecting an appropriate place to plant the tree take into account its final full grown size.
Measure from the bottom of the plastic wrap to the top of the soil. That is how deep you want to dig your planting hole. Planting trees too deep will lead to major problems down the road. The trunk flare should be just visible once the tree is planted. Dig the hole twice as wide as the root ball that is currently wrapped in the plastic.
When putting the root ball into the hole, remove all wrapping material, tease out any roots that are circling the root ball.
Seeing as you said your soil isn't that good, I would mix the existing soil from the hole with the same amount of good quality compost, then use that mixture to back fill the hole. When back filling the hole, make sure you heel it in.
With the leftover soil make a watering doughnut (approx 100mm high and 1000mm diameter) around the tree.
Fill the doughnut with 50 to 75mm of mulch (personally I would use good quality compost), start approx 100mm to 150mm away from the trunk* of the tree and work out to the watering doughnut. Using compost as a mulch in this situation has the added benefit of feeding the tree naturally and slowly. Then add a fresh 25mm to 50mm layer when needed (usually once a year).
Then fill the doughnut with water.
For the first year you will want to water at least once a week (you don't want to over water, but then again you don't want the soil drying out).
If the tree is planted in a windy location, you might have to stake (brace) it for the first 2 to 3 years.
*You never want anything covering the trunk flare. The trunk flare needs to "breathe". A sure way to make a healthy tree, unhealthy (eventually killing it, if the problem is not rectified) is to do something like volcano mulch.
Addition:
Today (2011-07-18) I was made aware of an excellent, but simple watering guide for trees, posting links to it here (see below), as I believe it's useful & relevant to this question:
Today (2011-08-07) I stumbled across the below article, posting the link to it here, as I believe it's useful & relevant guide: