There are lot of online resources! Here is an example: http://betterposters.blogspot.com/
If you browse some random articles, you'll see a collection of general poster making tips as well as some design hints. I think the best articles are the ones where other users submits a poster for critique. But remember that design is a subjective topic, so it's important to separate the "objective" stuff (important for clearly conveying information) and "subjective" stuff (something that is pretty to one person may be not to another, but probably good to stick to standard design "rules" for a first poster).
For software, I think the very best is Adobe Illustrator or something like that. But it's expensive if your school doesn't have a license! I usually use Powerpoint to make my posters (enlarge both the page and slide sizes). In Powerpoint, you would end up exporting as a PDF and when you do so, you need to make sure to embed all the fonts you use! In different versions of Powerpoint, there may be some issues printing the PDF, especially with margins. So make sure you set those correctly -- always better to have a little white border around your poster than have the edge cut off, in my opinion.
For printing, if your department has a large format printer, then probably use that? Make sure that the paper is actually big enough to print a 44x44 poster on (that's the AAS max size, not the size you have to print). I find that Staples will also print posters very nicely, but something that size can easily cost $80-$100 (they charge per sq. foot). But a lot of Staples only have printers up to 36" wide, so you should call and check before you design a poster.
Finally, for sizes of plots etc. my friend taught me an approximate rule of thumb that a size 72 point font is supposed to be roughly 1inch tall, so something like 36 pt font is a half-inch tall. You can experiment with printing out just portions of the plots and text at full size (on regular printer paper) and then taping it to a wall and standing a few feet back. I find that most poster sessions have quite narrow hallways so you don't need to be seen across the room! But it's probably a good idea for your text and figures (especially axis labels and legends) to be legible at a distance of 6 feet or so. And, remember that on your computer screen, contrast is much higher, so make sure you print out a 8.5x11 version of your poster once you picked the colours to make sure that the contrast on printed paper is still high enough to be readable.
There could be so much said about posters, so make sure you read a few articles on that blog above! Look up some sample posters online to get a feel of how scientific posters are done! And get feedback from your colleagues!