RvR at those levels is nothing impressive for you as well since your abilities are not enough and you lack most important ones. Simply nuking is never a good option for a Magus and you are forced to do that at those ranks. Still, if you have a good healer, Infernal Blast and Daemon Lash is a nice thing to do. Otherwise find a nice spot and nuke. Even Glean Magic is not something really important since most enemies lack resists.

II) Tier 2

Things get more Warhammer Gold interesting here, as you slowly get your important abilities as well as your first tactics. The mastery points you get at this tier are not enough to get anything gamebreaking, so I suggest you just stick with Changing for both PvE and RvR, since they are the strongest and most useful ones generally. That means that you should focus on using changing abilities (like flamer instead of other daemons) but the difference is not really big yet.

For PvE, after getting Daemonic Armor, things become much easier. I remember that I was able to kill 3 enemies of my level even before getting Pandemonium by putting Glean Magic, Baleful and Infernal Blast on them all, rooting them, running a bit back and killing them one by one with Rend Winds. After getting Pandemonium things get even easier since it does good DPS on many targets and benefits from Glean Magic as well. I suggest you pick the first changing tactic (Infernal Pain) as soon as possible and using it until you can get Endless Knowledge and together with Endless at 20.

For RvR, these are actually nice Ranks. Through them you will get some of your most useful spells, like Surge of Insanity and Warping Blast, as well as your main AoE DoT, Pandemonium. Using same Mastery and tactic choices, you can deal some serious damage that is still relatively hard to heal since healers lack AoE heals until level 20. Just make sure you try to interrupt, knock and root people as much as possible and rely on Rend Winds for single target damage and you will do well.

III) Tier 3

Things start getting Warhammer Gold trickier here. PvE and RvR ask for different specs and you actually need to start focusing on a mastery in order to be succesful.

For PvE, at 21 you can spec Demonology and get Agonizing Torrent. Combine it with Warped Flesh and Endless Knowledge and you can begin learning why leveling a Magus is easier than leveling any other class. Just do AoE pulls in your quests and kill as many mobs as possible. If you feel like grinding for an influence reward, just find a nice Public Quest with a long phase 1, set up your Blue Horror, pull 5-6 mobs with whatever way works with you, cast Agonizing Torrent, Glean Magic, Infernal Blast, (maybe) Pandemonium and spam Daemon Lash. Can make even bigger pulls if you feel like risking it a bit and using your Root and Daemonic Infestation to buy you some time. This is simply the best PvE spec you can have for rest of your ranks up to 40. At Rank 26 you can get Aegis of Orange Fire as well which really boosts your survivability and helps a lot with killing. I recommend skipping Daemonic Pact until you get your third tactic slot.