Earth main is basically like having a steel wall in front of your team, he won't really die and he just sits there and smacks his hands to give shields and sometimes jumps at the opponent. Earth mains main weakness is debuffs basically, ignites and poisons go through shields, earth main doesn't have a way to mirror reflect debuffs or remove them in any way.
About passive skills and attacks:
Unless you are doing 9tails event or playing a weird gimmick team, always use petrifying jutsu (The, give shield to 2 targets) and Steel Body, these 2 combined together makes earth main one of the best tanks in the game.
Also, always use Pose Of Duel, bottom force is honestly not worth it(Earth main will most likely die last anyways) and earth enhancement can technically give a little more damage but generally Pose Of Duel is better since there aren't that many easy access great earth based ninjas.
What mystery you use is up to you, all of them are good depending on your team. Although most people at your level usually opt for the middle one, Super Stone Fist Jutsu. Later on in the game the last one, Super Lightened Boulder Jutsu becomes way better.
A few teams that are good with earth main, (according to your level) are:
http://image.prntscr.com/image/c275797bc9214a7ab2bd260d2a7b717f.png
You will get tsunade at lvl 69 from plot instances, this is a pretty * good midgame f2p team.
Tobi does tons of damage, tsunade heals and sometimes trolls with her crit chaos, and hinata acts as a secondary tank + accupunctures when you do a chase combo. You probably need the correct summon to make the most out of the chase combos for this team.
http://image.prntscr.com/image/18359e3693964c0f971023f0d6deaae7.png
If you managed to get Karin at this point this team is pretty good, it offers less survivibility but you get poisons + ignites from karin/sasuke.
You could also replace Karin with Kabuto for this team in case you haven't managed to get her.
Of course it all depends on what ninjas you have, if you were able to get a Pain ninja, it's probably better to use that in a team.
Or if you were able to get ninjas like sage jiraya, windblade asuma, iruka etc. those are probably better to use.