As a beginner, you should start here
I highly recommend the video copilot series and Angie Taylors videos.
There are also 2 entire basics series for AE, HERE at the Cow
AE FAQ: Why is my output file huge?? The short answer though is After Effects renders files to uncompressed “AVI’s”. If you want to compress your files AE can do it, but it’s not the best tool. The general work formula is to take that uncompressed file to media encoder where it can do a much better compression job.
Here’s a from VideoCopilot that shows one of creating the smoke/wipe you are trying to achieve, but it’s may be a bit advanced. If you to a google search for “create smoke after effects” you will find a lot of different recipes for creating smoke(there are a ton)
Good Luck
Johnny Cuevas, Editor
Thinkck.com
“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.