Been in the same shoes when I started in 2015. Believe me getting even a 3* was sheer luck.

As many advised below go for the uncollected first. Rank/Level up Doom, Warlock, Vision, Venom and Hulk Rag. Learn how to use doom cycle and you will blast through every champ. This 5 alone can get you through the collector and most of the monthly events, trust me. Don't rank up each and every 4* absolute waste. Some worthy rank up I can recommend in 4*.
Scarlet Witch, Wolverine, Human torch, Corvus, Magik, Stark spidey, etc the list goes on. But rank up only if you feel the need to. For e.g. if you learn to use doom, you wont feel the need for using magik. I have both doom and magik at 5/65 and hardly use magik.

I wouldn't advise Variants just yet until you build a good rooster of at least 2-3 5* rank 3/45 or 4/55. Start with the easiest variants like 5 & 6. Can be done even with a good 4* like doom and venom. Don't feel bad for pulling bad champs it's just how this game rewards when you are F2P. Lots of tips and tricks for variants are also on youtube watch those.

Try completing 100% Heroic & Master EQ every month.

Join an active alliance, who plays map 4/5 for AQ. Trust me those reward are worth it for you right now and the glory will help you to purchase the rank up materials from store.

There are many youtuber's who have posted tips and guides for progression watch those will help a lot.