Buy plenty of eliquid to start. People are most likely to run out of eliquid when they first start. Later you end up with bunches of flavors you don't like or vape much but you can vape them if you get low on juice. I recommend you try PG/VG blends around 70pg/30vg then if you want more TH try 80pg/20vg, of you want less try 50/50?
If you vape out of one 3-6ml bottle to start then you will be able to look at the bottle and see how much you used up in 2 days, in 3 days, etc and know how much liquid you vape per day.