1) After I finished my model, I went to the upper left hand corner where I choose the drop down menu on the Inventor Logo and hovered over Save As, if you hover it pops up a secondary menu where you should choose save a copy as. In that menu you will want to choose .stl for the file type and then click save.
2) You can download 123D make at http://123dapp.com
or choose to upload your file to the site. From here 123D make is pretty self explanatory. I chose to import my file and then chose stacked as my creation option. From there its up to you on size and material thickness. The laser Cutters at TechShop are 18x24, so that is typically what I would pick for the sheet size.