First time you click the arena button, the innkeeper spots you the gold needed to enter, so you get a free run. Do some research and practice games first before you use the free arena though.
There's also quite a few relatively easy achievements you can do right off the bat for gold. Check google for those.
And be patient. Unless you're willing to spend real money on cards or adventures or spend many, many hours playing, deck progression is kind of painfully slow.
This post was edited by chemoshots on Jun 7 2015 09:46pm