Use could for ability or possibility, would for willingness or an intended result. You could try the new cafe means it is an option; I would meet you there states the plan. In polite requests, both work, so pick by tone.
Paired examples
If I had more time, I could learn Spanish. Spanish is one possibility among several; time is the only thing missing.
If I had more time, I would learn Spanish. The speaker has already decided; given time, learning Spanish is the intended result.
I could swim when I was six. This reports a past ability, something the speaker was able to do.
I would swim every summer. Would describes a repeated past habit, what the speaker regularly did, not what they were able to do.
Could you send me the file? and Would you send me the file? both work as polite requests asking for the same thing. The choice is one of tone, not meaning.
One caveat worth knowing
Most polite requests accept either form. Could you send me the file? and Would you send me the file? both ask for the same thing, and neither version changes what the speaker means.
The difference is only in how each one lands. Could frames the request around possibility or ability. Would frames it around willingness, asking whether the person is open to doing it.
Could you fix the printer if I can't get it working? asks about possibility, whether fixing it is something you might manage. Would you fix the printer for me? asks about willingness, whether you are open to doing it. Since the meaning is identical in most polite requests, choose by tone and keep the one that sounds like you.