Hey everyone, I'll write this as a general guide for FTP accounts. I am F2P since day 1 and have made my fair share of mistakes, so hope this guide can help you avoid some of those.
Here are some tips:
(1) Save your crystals for refreshing dark side/ light side missions and cantina. While it might be tempting to roll the dice for a nice 4* character, you're MUCH better using the crystals to focus on your characters.
(2) Only farm one character per stage at a time. I think the biggest issue players run into is they aren't sure who to go for, end up picking up a lot of lower star/ gear toons, and end up lacking an elite squad. You should use most of your light/ dark side energy for farming gear, which is incredibly important. Doing so allows you to focus on 5 characters to build your first team (Cantina Shipments, Cantina Shards, Light/ Dark Side Shards, GW and Arena Shipments). Try to plan a good team synergy with the characters available from these 5 locations, since yo can realistically only farm one at a time.
(3) Good characters to farm...
GW: Luminara, Phasma, Poggle, IG-86
Arena: Sidious, IG-88, Ventress
Cantina Shipments: Poe, Dakka, QGJ
Cantina: Kylo Ren, GS, Luke, JC, Dooku
Light/ Dark Side: Any luxury character you want... It takes much longer to farm these shards.
There's other really good team synergies, but make sure you have an idea of the team you're trying to create before you start farming.
(4) If you're just starting the game, it is worth investing some into the starting characters. You will want to start building towards a more elite team, but having a few 4*/ 5* characters will help you win arena battles and GW to start the building process.
If you can get to the 50 - 20 ranked place in arena, you should earn enough daily crystals refresh you l/d side missions twice and your cantina missions once.
(5) Don't complete missions just, "because". Figure out the characters you want to farm from cantina and l/d side battles and only complete the missions required to farm the toons you want. You should spend as much energy as possible farming those toons and gear for them.
(6) Potential teams: F2P accounts have easy access to strong droid teams, and can make elite First Order Teams with Phasma and Kylo. I don't think any of the starting characters are worth taking to 7*'s (maybe JC), so don't be afraid to stop farming your 5* Chewy (for example) for a better long term character.