First of all: Don´t worry about that 5 star heroes are the least important of them all in the beginning of the game where you are. Ignore them all, no matter if they are good or bad. By the time you are ready to think about 5 star heroes, you will have better ones anyway
Focus on your 3 star heroes first. Max 4 or 5 in each color. When you get the materials you max a 4star you can throw one of those in as well, as you have already done.
No matter which hero you choose, max it! Stop wasting materials on half upgrading heroes! Max one, then move on to the next. If you don´t have materials, max a 3 star!
There is not much point talking about teams while you have no heroes maxed at all, just use whoever is upgraded highest for now on defense and on offense stack the strong color against the tank (using 3 or 4 heroes of that colour and a healer in another colour)
AS to which heroes to work on: First the ones that are close to maxing: Orla, LiXiu, Poppy, Zarola, Boril, Ulmer, Jarif
Would help to see the rest of your 3 stars, but some that are most certainly worth maxing: Yao, Helo, Fianna, Planchet. But also the others are useful, especially when you don´t have lots of them yet.
4 stars worth maxing when you have mats for them: Ferant, Proteus, Hansel, Cillian, D´Andre. Grimm and Tiburtus are good because of the defdown (and because they have costume bonus). Also all the S1 healers (especially if you have their costumes), Wu Kong is great for titans. But as I said, only work on them if you have the mats.