The most important thing is to make absolutely sure that fermentation is done to completion before deciding to package. Depending on if it's straight cider at 5% or you added sugar to bump up the ABV, and also depending on how nicely you treated your yeast, this could take anywhere from 3 weeks to a couple months. Dave Taylor, whom I really trust when it comes to cider, recommends racking off the lees (sediment) every time it reaches 1/4 inch thickness, until it's not producing anymore.
I like to use frozen apple juice concentrate for carbonation. I use 1 can for a 5G batch, so for 1 gallon, cut it down to 20%. Others prime with a little sugar as you would a beer. Bottle, keep warm for 2-3 weeks, then chill. Cider gets better and better over the following year. Don't drink it all in the 1st 2 weeks (although with only a 1G batch, it'll be hard to save any).