Step 1: Materials
For now let me just state the basic components of a marimba and the materials you will need.
The Bars - this is where everything begins. The bars can be made from nearly any material, but to qualify as a marimba it must be wood. Feel free to experiment with different types of wood before construction. But it is important for the wood to be completely dried out (not green at all). My oak material came from a tree which was struck by lightning.
The Frame - for me, this was the next step after building the bars. The frame can be made from anything. This includes wood or even steel. Use whatever you are comfortable with.
The Resonators - Nothing difficult here. Though anodized aluminum is very pretty PVC pipe works just as well.
Those are the basic parts of a marimba but you will also need some specialty equipment.
Musical Tuner - How much you invest in a tuner will be reflected in your marimba. If you just want something to practice with (like me) then a 30 dollar tuner will do just fine. Otherwise, if you want to tune overtones, use a strobe tuner (300+ dollars).
Belt Sander - You will be using this a lot so get something comfortable.
Drill - You will need to drill holes through the width of the bars for the marimba string. I suggest a drill press but a hand drill will work just fine.
Table Saw - for making all those cuts.
Band Saw - not essential but recommended if you will be cutting bass notes.
Miter Saw - really handy with the frame and resonators