1.) Open all packs before spending any gold.
2.) Go to "my collection" click the tab for card crafting.
Once all your packs are open then click on the tab that says "Mass Disenchant".
(You want to gather all the Arcane Dust you can get.)
3.) Once you complete that step, and then you can go in an buy the current decks witch will be the newest cards for this season. You can always back track later, but if you are gonna play wild deck then it don't matter.
Playing with the current ranked (non-wild) decks you will want current cards. Current set is "Perils In Paradise" Yes there is a lot of good cards from last season etc... Try to focus on coming back with the cards that are current.
The set "Whizbang's Workshop has a lot of good cards that would really come in handy as well. Try not to get over whelm about it at first.
4.) Research current ladder decks. Try build a deck with characters that you hold the most amount of cards and with your dust you can use to help you complete your legendary collection for your deck that you decide to build.
This would be just a short summary of how I would go about it.
P.S. If you manage to open everything that you currently have. I would also recommend going to the shop, go to Hearthstone tab its the 2nd tab down. Then click on the mini-sets. You can see a mini set with 72x cards you can buy that cheaper then buying the gold set. This will help a lot.
You will see all the mini sets there. Whizbang's Workshop, Shadow in the Badlands, Titans etc... Just remember not all cards will be able to be used. A lot gets tossed back to "Wild", but some will change from time to time.
If you need anymore help fill free to pm me.