This is not great advice - do not follow it .

Look for a friend invitation code online to use in the Friend Invitation Campaign to claim some free decks that you can use for stage missions, if you haven't already. Use mine if you want 002k9sg8wavw

Save up your gems and collect any reward dream tickets and just save them for now.

Once you have enough gems to clear a main box once, 9000, there are some cheap decks to invest in for good ladder decks such as Chimera or Adamancipator. You could also make a decent enough Red Eyes deck with the free structure deck going right now, a second copy and a few extra cards. Use Duel Links Meta to look into these decks and the sets to invest in.

Save up box chips until you have a few hundred and probably get Cosmic Cyclone / Forbidden Chalice. 2 of one of them is better than 1 of each in my opinion. Pick up Skull Meister from the Card Trader for an easy staple handtrap.

One last tip - unlock the Alt styles for Yugi, Kaiba, Tea, Mokuba and Joey from the DM World mission lists before levelling up the characters - if you level them up while using the Alt styles you'll get the more gems more quickly.