I disagree with the notion that playing freerolls will improve your game. CC freerolls? Possibly. But most freerolls are filled with donks that can't wait to shove their whole stack with 6 high.
My best advice to improve your game would be to STUDY. If you have a few dollars to invest, deposit it and play the micro cash games for a while and keep track of your play. Do you find yourself getting bullied around after 20-30 hands? Are you only playing the top of your range? You need to understand ranges, position advantage, player tendencies, pot odds, implied odds and overall hand equity before you can truly understand why you should be doing a certain action in a certain situation.
If you don't want to inevitably lose $20 on the micro cash tables, look up some content on here, Upswing Poker, PokerNews, etc. There is tons of information available from Poker Pros who are crushing the tables. Info such as when to call a 3bet, vs when to 4bet or fold, why blockers are so important in 4bet pots, and so much more. It takes time, but STUDYING and HANDS-ON PLAY are what is going to improve your play as you learn from your mistakes and fix any leaks in your game.