In the Ranked Constructed deck selection screen, MTG Arena only displays your legal decks there (containing at least 60 cards and no more than 4 of any card other than basic lands.) So if you can't find your decks, first go to the main Decks scene in case your deck is not yet legal for Constructed.
If you still can't find decks you've made there, you may be impacted by a bug which has the following workaround:
- Close the MTG Arena client
- In Windows Explorer, go to %USERPROFILE%\AppData\LocalLow\Wizards Of The Coast\MTGA\
- Delete DeckPlayedHistory.txt
- Open the client - your decks should be there now.
If you are looking for the 10 2-color preconstructed decks, those are unlocked by completing quests following the tutorial. If you don't have all of those decks yet, keep completing quests to earn them.