Hey, took me weeks to figure this all, (Which isn't my field anymore but I took some months on CPA)
CPA = Cost Per Action. You get paid when someone completes an action. Like submitting an email, installing an app, or filling a form.
CPS = Cost Per Sale. That means someone actually buys something through your link. Usually it's harder to do, but pays more.
Landing page - Not absolutely required but it helps a LOT. A landing page warms up your traffic and boosts conversions. You can start with free tools like Beacons, Carrd, or even a simple WordPress site.
Do you need to pay? Depends. You can start with free organic traffic (TikTok, Reddit, YouTube) or paid traffic (Facebook ads, Google, native). Paid traffic requires a budget and testing. Organic takes longer but costs nothing upfront.
I would advice you picking one source traffic and learning it well, pick a niche or offer to focus on. Based on that build a landing page to preframe your offer, and test creatives and angles constatly.
Of course, even if through ads, don’t expect to get rich overnight. Unfortunately CPA requires patience and testing, but it’s definitely still "legit" in 2025.