Solo Arena is a great place to start building up a collection. Not too difficult and great fun to play. You can do ranked too but it can be frustrating if you have a small collection. Versus Arena is ok if you are a Hearthstone veteran but it gets old real fast for me.
For constructed, it seems that yellow cards are the best when you’re starting out. You get one of the primary epics for a token build early on (Divine Fervor) and many of the best cards for the deck are commons and rares (Imprison so good!). It also has room to grow and you can upgrade cards as you get more resources (Pit Lion, legendary cards).
EDIT: Here is the deck I am working towards, with my planned budget replacements included
3 Deadly Draugr
3 Scouting Patrol
3 Fifth Legion Trainer
3 Wind Keep Spellsword
3 Kvatch Soldier
2 Siege Catapult (budget: Bruma Profiteer)
3 Pit Lion (budget: Imperial Legionnaire)
3 Eastmarch Crusader
3 Haunting Spirit
3 Young Mammoth
3 Divine Fervor (budget: None. This is the only epic you need to craft)
3 Midnight Sweep
3 Imperial Reinforcements
2 Edict of Azura (budget: Bump cards with 2 copies to 3)
3 Piercing Javelin
2 Shadowfen Priest (budget: Vicious Dreugh)
2 Golden Saint
TBH I would play three copies of Siege Catapult and Edict of Azura if I could, but they are the weakest cards in the deck in a meta full of tokens/archers which is what I see most on Ladder. Arrow Storm might be a good tech card in such a meta but I am not sure if being more proactive is just better. There are a lot of players with Red/Blue voltron decks on ladder too but I wouldn’t recommend those as they fold to a single removal spell which token and archer decks have plenty of.