Welcome to KH and the wonderful, wacky world of knitting.
It sounds like you're tackling a biggie for one of your first projects. Very impressive, indeed!
For something that large, though, you may very well need to use circular needles. There are, thankfully, some tips you can learn that will help you keep your ends straight:
At the very beginning of a new row, your working yarn should always come from your left needle (like it normally does with straight needles). Then once you get going, your working yarn will be coming off your right needle (again, just like it does with straight needles). One thing you can do to cut down on confusion is to always finish a row BEFORE you set your work down to do something else. And if you have to set it down mid-row, use a sticky note to mark the direction you should be knitting.
Once you finish a row, you will move the needle in your right hand to your left hand and the needle in your left hand to your right hand. This is exactly what you do with straight needles, but for circs, you just pretend that the cable connecting your needles isn't there.
Also, be sure to keep track of where you are in your pattern. That should help you figure out which side of the afghan goes towards you and which side is away from you (i.e., if you're on a knit row, knit stitches will be facing you; if you're on a purl row, purl stitches will be facing you).