For your first backpacking trip, you should probably stick to a simple overnight excursion. After that, you can progress to two-night loops and eventually to one-way hikes, which require you to find a way back to your start, but let's save those for later.
For your route, you'll want to consider a few things. Firstly, you're carrying more weight so you won't travel as fast as you do on day hikes. Aim for about 60% of your longest day hike per day. Sure, you might get to camp early, but that gives you more time to set up your new tent and build a safe campfire.
Secondly, think about the elevation. You don't want to be going all uphill on day one and all downhill on day two. Instead, look for a rolling route.
When picking your campsite, the first consideration should be legality. Make sure that wherever you are planning to camp is legal and that you have all the required permits. In most national parks, you can easily get a backcountry permit but be sure to carefully read the rules and obey them. If they say camp 50 yards from a trail and 200 yards from water, do that. You don't want your campfire cookout spoiled when a ranger tells you it's time to move.
Water is important, and you don't want to be carrying all the fluids you'll need even for 48 hours. Plan to camp somewhere near water. If possible, you might want to look for a campsite with numbered sites, sure they cost a bit more and aren't quite so rustic, but you'll know there is nice flat ground to sleep on and running water.