Depending on what the shield looks like, and how much work you want to put into it, you have a couple of ways you could go.
There are 2 basic techniques for forming metal; Raising and dishing.
These 2 basic techniques have variations of type of tools and type of surface you work on.
Raising is compressing metal, while dishing is stretching metal. This can be done with the metal at room temp, or hot. Hot has many advantages.
Dishing seems to be where most people start because it seems more intuitive, and the tools can be a stump, and a basic hammer.
But raising, which requires (generally) a hard metal surface (anvil or metal shaped "stake"), is perhaps more universal and can provide more control over material thickness and shape.
As mentioned above, metal spinning (see youtube videos, it's pretty amazing what can be done.) is a form of "squish raising", but require complex tools. Squish raising is where your metal is hammered or pinched between to harder surfaces and is squished out from between in a desired manner. Imagine taking a flat sheet of modeling clay and poking it with your finger, while the clay sheet is on a desk top. Each dent you make with your finger, displaces the clay. If you do these "dents" in a spiral motion starting at the middle, while slightly lifting the edge of you clay sheet, you will eventually make a cup shape.
Using an "English Wheel" is another form of squish raising, where the metal is pinched between to hardened steel wheels while being rolled back and forth or in and out from between the wheels. The is a good way to make gentle curves and large plates. Used mainly in the automotive or aircraft industries for body panel manufacture.
You can buy metal shape that have been spun, either in aluminum or steel, sometimes brass and start with this and do additional hammer work.
Here are some examples of items made in metal, using raising techniques.
First is 16th century helmet made from spring steel, "still rough from the hammer", meaning it still need grinding and polishing. (Made by Jeff Wasson)
Second is a fantasy 16th style 1/2 armour, in steel, but repoussed with small tools and chisels. (Made by Ugo Serrano)
Lastly is an aluminum Iron Man helmet, rough from the hammer. (Made by Greg Wosnak)
There are some good youtube videos if you search "metal raising", showing techniques using steel, aluminum, copper and silver.
If you search "metal forming dishing" you will find some good videos on dishing, while others are not the best. If the metal piece is dished and the resulting piece is relatively smooth, then it's probably a good tutorial. It the resulting part is all lumpy and out of shape, look for another video...
Good luck