Hearthpwn

First, make sure you are completing all the hidden quests. Look at them HERE

Now when it comes to what to use your gold on, the adventures are AMAZING value but make sure you're just getting the most recent ones, definitely League of Explores when you can and MAYBE Blackrock Mountain if you can. Before that you should probably get some cards to compete, i would recommend CLASSIC packs since they are generally more "useful" and will never leave standard but don't hesitate to play ARENA to get better value if you win more yet you can only get the newest expansion packs, which isn't terrible. If you aren't getting 3+ wins in arena it's better to just buy CLASSIC packs and sometimes the new expansion packs. Hope this helps, good luck!