I want to do cpa and eventually get to media buying. I have about $500 now to start and about $125 a week to spend. What would be the best way to start?
Very low payout offers (e.g. mobile app installs).
Cheap traffic (low CPMs, e.g. South-East Asia, South America, Arab nations).
Traffic source: something with a low barrier to entry. If doing mobile we recommend Decisive due to the data transparency but you could also use TapIt! for example. You could also try Facebook in cheaper countries.
Read as much of the forum as you can handle (arguably the most important thing to do). If doing Facebook, read my guide back to front (linked in sig).
Write down a clear plan before starting a campaign.
Start it as a follow-along.
Focus on not profiting, but learning as much as possible from the data you have bought.
Be aware that with only $500 you will struggle to get enough revenue moving to get paid quickly by any affiliate network, so you should plan for the long run - i.e. expect to get any revenue back some 6-8 weeks after starting.
Good luck!
Before starting spending any money, learn as much as possible from the forum. See what mistakes other people have made, see what fundamental knowledge you need to develop. Think of this as tuition.
Only start spending money after you feel like you at least have a basic grasp of how things work.