Welcome to the forum, MMM! As these fine modelers have said, you can start with either a 3:2 or 2:1 paint-to-thinner mix ratio (specially for oil-based enamels). If you wish to be sure that it's a good-enough consistency, follow this visual check which I got from a Tamiya painting guide: after mixing the paint and thinner, lift the paint stirrer or mixing stick off the liquid and watch how the mixture drips from the stick. If it drips continuously in a thin stream as if you're pouring pancake syrup, then it's still thick, and you need to add more thinner. If it drips in drops, then it is good enough for airbrushing. The faster the dripping, the thinner the mixture is. Since you're just starting to AB and are not yet into detail work, I suggest that you avoid mixing your paint too thinly so as to avoid paint runs.
Take note that most water-based acrylic paints are already thin and can even be sprayed straight from the bottle, like Tamiya Acrylics. Usually they require only 10-20% thinner. But whichever type of paint you're going to use, I suggest that you practice AB'ing first on a scrap styrene sheet or model, and experiment with spraying distances, angles, etc. That way you'll get the hang of AB'ing and see what "surprises" or problems may come up when you AB the real thing. This will also save you the frustration of possibly botching your first AB job on a good model. Good luck and hope this helps.
Cheers,
onyan