Here is how to build a relatively easy redstone trap.
Dig a hole 5x5x2.
Put sticky pistons against the edges of the hole, don't do the corners it's pointless, then on the inside of the sticky pistons, use redstone dust to connect them.
Fill it all in with whatever block you want that can be pushed.
Put pressure plates one in from the edge, even though you avoided the pistons in the corner still put pressure plates on the corners.
Put 3 high pillars in the corners, then make a roof, make sure you can't jump inside the structure.
Put a chest in the middle and fill it with whatever you want, I used diamonds.
There you have it. A trap. Make sure the pressure plates match the floor, I used something different so you guys could see.