Which metal to place at cathode and which one at anode?
Answers
The different metals used for cathodes and anodes are not for electrolysis but are for acid batteries (and are usually zinc & copper).
You don't need specific metals for electrolysis, just a metal that conducts electricity, and the two electrodes don't need to be different metals (as they do if you were creating an acid battery).
The important part is having a solvent mixed with the water. (Pure water is non-conductive, so won't work.)
A little chemistry knowledge will help (polarity of hydrogen vs oxygen to identify which electrode is attracting which element).
Also be aware that certain metals will react differently in the electrolysis reaction, combining with either the oxygen or hydrogen to produce new compounds (and in doing so, corroding your electrodes over time).
For example, when I was in 6th grade I used a home-made electrolysis rig to split baking-soda water into 2 parts hydrogen to one part oxygen.
I also used large chunks of copper for my electrodes which meant that on the one polarity, some of the oxygen combined with the copper and created copper-oxide. Later I collected and concentrated the copper-oxide and used that as my solvent in the water. I then by clipping a quarter to the opposite electrode I reversed the reaction and copper-plated the quarter.
Science fun