Okay, first thing's first. Under the model tab, chose "select" and make sure the box at the bottom that says "ignore backfaces" does not have a check on it. That way you can select more of your model at a time. Click on the "face" button right above that.
Second, click and drag a square across the area you want copied, the shoulder.
Then go to the edit menu, and click on "duplicate selection" Then click on the vertex menu, and click "Mirror Left <--> Right"
It's mirrored.
First, make sure that the other side does not have any polygons where you will be copying it. In fact, it's best to only make one side of the body. the left half of the chest, the left arm, the left leg, and the rest, then in the very end duplicate and mirror. Make sure and hit "weld all verticies" under the vertex menu afterwards to make it a solid object. Also make sure there are no polygons at the seam where you will be mirroring.
Now, as for a general rule, only put polygons where you need to change directions. For instance, if the waist goes straight up, you can make all the polygons across it be very tall, going right up to where the chest is. If you need to have them be able to bend forward or back, so their waist won't stay flat, you need to ad a row of polygons.
There is a term used in the industry called "edge looping" It is basically making natural curves or lines using the sides of polygons. On a muscular, shirtless man, there would be a line down the middle of the chest, then it would branch off to each side, curve up to the armpit, then back towards the neck, creating the inside edge of the shoulder muscle.
The problem with your sword is that it is not edge-looped. A flat sword with a sharp edge doesn't need very many polygons. A line down the middle, two triangles making up each side's top, then two more for each side's bottom. There, you've created one with an edge on each side (but no tip. For a tip, add 1 triangle on each side's top, then bottom)
I hope that helps.