You might need some help from an adult because you're going to have to use a table saw or jigsaw.
You can either make it out of wood or cardboard i like cardboard because it can be more detailed,though making a gun out of cardboard is mostly improvisation so there isnt really any specific way to make one becase you might make it hollow or you can glue a bunch of cutouts together.

Step 1: Things You Need

You will need:
wood
a table saw or jig saw
a marker
a copper tube-should be about an inch longer that you want the barrel to be
help from an adult
a picture of the gun you are trying to make
a glue gun with glue stick cartridge things
paint-spraypaint and maybe some acrylic paint
thin cardboard-i used the cardboard from a cereal box
these are the basic things you need but ill mention some other things you can add and how i added them.

Step 2: First of All...

First you are going to draw the basic shape of the gun on the piece of wood or cardboard with the marker.
Make sure the lines are straight and that its scaled correctly.
If you want the clip to be detachable, don't draw it connected to the rest of the gun.

Here is my design drawn on the wood.

This isnt the same gun as in the other pictures.i was going to make this one but there was a knot in the wood and it would've cracked.

Step 3: Cut the Wood

Ok now heres were you might need help.
you're going to need to cut the wood on a tablesaw.

well this step is the shortest but actually cutting it takes a while.

If you are using the "layering" technique, just cut out a bunch of outlines of the basic shape of the gun and glue them all together.

Step 4: Add Details

For details of the gun that stick out slightly, simply cut them out of thinner cardboard and glue them on. You can do this whether you are making a gun out of cardboard or wood.

Step 5: Scope and Silencer

For the scope i just hot glued a binocular onto a piece of wood and hot glued that on to the top of the gun.

The silencer is a small PVC tube painted black.To make it fit tightly , i wrapped the end of the barrel with tape.

The trigger is just a piece of plastic.I drilled a hole in the wood and hot glued it in.

and thats about it...

Step 6: If You Are Making One Out of Cardboard....

If you are making one out of cardboard keep in mind not to use so much tape because evern after you paint it ,it will look glossier than were there is no cardboard.I suggest you use hot glue and only tape on the inside but If you do use tape i suggest you use masking tape because it is more porous than duck tape so it wont look as glossy.I didnt have masking tape so i just used duck tape.
UPDATE:
sommeone asked how to make it out of cardboard so heres pretty much what I do.

1 I drew the basic shape of the gun on cardboard and cut itout then i traced another one of it and cut that out.
2 I cut out a couple of strips of cardboard that are about 1 1/4"wide and hot glued them to one side of the cardboard shape of the gun.Once your done making the sides of the gun glue the top on
3 Add details made out of hinner cardboard
4 for the clip to be detachable you just dont put cardboard on the sides were the clip would be then make a clip the same way you made the gun and it should fit in the hole without falling out.
5 paint it
6 for the barel i used a piece of thinner cardboard that i made into a tube.

thats about it