A little dated 'cos its from 4 months ago, but most of it is still relevant:
https://www.reddit.com/r/future_fight/comments/4w8bsf/f2p_tips/
Take note of Point #5 about cards especially. Point #6 is contentious nowadays - Proxima is still very good but for a brand new player, you may find yourself getting more mileage out of T2ing top tier characters. Ditto for Point #4 - with so many new top tiers, Loki may not always be the best choice. Still very good, but not the best. The immune damage mechanic trumps Loki's guard/shield.
To add to that, you should T2 your Sharon to carry you through all the PVE content, and help you score decently in TL (to earn crystals). If you are F2P, don't spend your crystals frivolously. Uniforms for top tier characters are usually a solid investment. If you cannot clear Shadowland 5, 10, 15 consistently, you may also wish to consider using crystals to speed up your progress on Dr Strange (i.e. buy selectors, 6 star tickets etc. to rank up the requisite characters). Dr Strange is the most OP character in the entire game at this point, and you should definitely make getting him ranked up your biggest priority. There are tonnes of Epic Quest guides out there - just search "epic quest" on this reddit.
Good luck have fun :)