I always point newer players to this:
Best advice I can give is to NOT level 5* right away. I did that, and it set me back months on game progress. 5* take a LONG time to level up when just starting out. Even starting right away it took me 10 months to max my first (I also play f2p – roughly).
Second, stay f2p. It is much better for your mental health 
3* should be a first priority. They are quick to max, do not require materials that you cannot get from map stages, and will help you get materials from quests fro higher tier heroes. Then move up to 4*. Brienne is a good 3*, she supports your other heroes by boosting their attack. Balthazar hits pretty hard. Karil is decent enough. NOT Graymane. A team or two of 3* will also let you participate in 3* tournaments to get more loot.
Save any 4* healers you get in future. For f2p 5* healers are a rare resource and you will use the 4* ones forever – especially Rigard and Melendor. I use Boldtusk all the time because I have no 5* healer at all.
It helps if you can find a good alliance to join that will help you out with advice too.
Most importantly, just have fun.