I think upwork is one of them.
But like any other tool, you should follow some tips in order to avoid fake clients or to use successfully what upwork offers.
What it’s good for me is the simplicity about how will you earn. They take 20% first 500 u$s , 10% from 500 to 10000 and 5 % after that… (for the same client)
There are some videos with English captions with tips about how to find (and discard) jobs there: Como Ganar Dinero En Internet con Upwork – Consejos Reales y Prácticos – YouTube