Welcome to the community and the game 
First thing I’m noticing is that you have very little room for extra 1 star and 2 star heroes. You’ll want some extra room so you can train your stronger heroes with them.
To make room, you should consider buying a hero cap increase under “Resources” menu in the shop. It starts off at 50 gems and eventually increases to 100, but it’s relatively cheap and a good use of gems when you start running out of room. You also get them as you level up, but level ups will start becoming more and more slow as you progress in the game, so these cap increases are great.
Next I would train some of your 3 stars. At least five of them so you can participate in some of the rare tier quests with the best chance. Start off with Tyrum, Bane, Brienne, Valen and Hawkmoon.
Your 4 stars will become the backbone of your defense team for a while, and your attack teams for a much longer time. The ones I would recommend training:
- Purple: Rigard, then Tiburtus
- Yellow: G. Jackal, then Wu Kong
- Blue: Sonya, then Boril if no one better is available
- Green: Melendor, then Skittleskull if no one better is available
- Red: G. Falcon, then Scarlett
Your 5 stars will take the longest to train, and it’s going to be a long time before they get all the items they need to ascend to tier 4. Also, you will need maxed 4 star heroes to get better chances of getting those items: from killing titans for loot, to participating in wars for loot, to clearing the quests that drop those items.
Keep the duplicate 4 and 5 stars you get. Keep all heroes you’ve trained so far as well… you never know when they’ll be useful. And if you want to try for high scores in event quests, keep the duplicate untrained Banes you have.
Finally, here’s a good topic I would recommend reading: Advice for New Players, or "All the things I wish someone had told me when I first started."