My 2003 MBenz C240 won't start... Battery is good. When I try to start, the engine turns over but won't start. Is there an easy way for me to troubleshoot the problem myself? Is there a link to a step-by-step guide to check spark plug wires, fuel, etc? Anyone's help is GREATLY APPRECIATED! Thank you!

Looks the RFDI chip in the key is defect. Your Mercedes car dealer should be able to help you with that.
Fist to make sure it is a chip problem in the key and second by replacing the chip and the code and / or replacing the battery it an empty battery is the issue.

auto or manual ?

CHECK ALTENATER

It is located on the transmission where the sghift cable connects to the transmission.

Did you check your battery???
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sbb key programmer

Try checking the key reader it's usually a plastic ring around the key barrel. If it's dies will not read code in key to enable the car to start. Check your key is sending a code as well.

replace the spedometer cable.

Checked Cables how?

Looking & Touching isn't doing anything

You have to do a voltage drop test or remove
& clean the ends. I would do both

Remove the starter & have it tested at parts store,
assuming you don't know how to do a voltage or
current test while cranking.

What do the hesdlights do if they are on? Gim, go out?

I am assuming that the engine won't even turn over. could be lots of different things. The simplist is to make sure the car in in PARK , won't start if not. Other things could be , Security imobilizer if you have one. Does your key have a electronic chip in it ? Try your other set of keys if you have one. Check if battery connections are clean and tight.