Well, usually the first job to take on is that of a Trader.
If you're a hunter or a thief at your low level, you will most likely be 1 hit by higher lvls just for the hell of it.
so doing traderuns with one star worth of cargo is probably recommended.
However!
you might actually want to wait a bit longer than level 20, on the way to Donwhang from the ferry, you will meet monsters ranging from level 19-28 (level 30 monsters if you travel through Ravine, not recommended without a hunter, unless you can easily handle monsters in there).
I know its kind of lame not doing a job when you can, but I remember doing a trade run first thing I did when i hit level 20, i got killed when i reached a pack of earth ghosts..
EDIT;
and yeah, there are TONS of other threads on this..
http://www.silkroadforums.com/viewtopic.php?t=2815
should suit you fairly well.
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dom wrote:
I never use more then 20~gb, and most of that is in porn alone.