Try asking one question at a time and be as specific as you can. The learning curve is high, but once you get it down, it's not hard.
The parameters for whatever you are doing are at the top of the screen, so if you are laying down traces, the width parameter is up there and you can change it when you are drawing. If you want to change it after the fact in the board editor, select edit->change->width->(select the width value you want) then click on traces and they will resize to the selected width.
The copper filling is called a 'copper pour' or a 'ground plane'. If you search for Eagle tutorials on those, it might help. Select the polygon tool. Draw around the edges of you board until the ends connect, select 'ratsnest' to fill it in. You want to rename the polygon with edit->name->click on the polygon edge and rename it to the same net name as your ground (GND, or VSS).
Do you need to use gimp? You can print right from Eagle. You can mirror it and make it full black/solid; the print function is made to be toner transfer or photo transfer template friendly. Make sure you use the display tool to turn off the layers you don't want to print.
EDIT: Oh yeah, if you aren't doing schematic -> board layout, you should. Trying to go just directly to board layout is really bad practice you'll want to do the process correctly from the start.