If you haven't read Sanhatlig's guide yet, you should. He's got a lot of basic information that helps cover all the bases.
The most important thing to do first off is to stack your Eidolon grid with the highest level (highest assault and hp you can get) Eidolons you have. Only your main has to be Light. All Eidolons add their hp and assault to all members of the team.
Next you want to get a weapon grid which is mostly assault. Defender is good, but secondary. The difference between Luminous > Shine > Ray is 3% base plus 0.5% per skill level. So a level 1 Ray assault is 0.5% assault and a level 10 Ray assault is 5% assault. Similarly a level 1 Shine assault is 3.5% assault and a level 10 shine assault is 8% assault. The luminous adds another +3% over shine weapons. This means that an assault SR (which are at most shines) has 3% less assault than any luminous SSR. Even though you can take an MLB (max limit breaked) SR to level 85 and a zero star SSR to level 50, it might make sense to use the SSR instead of the SR if it gives more assault. Total power for an SR is around 1800 and total power for a level 50 SSR is around 1350-1400. That means if you have around 15k or more in assault the luminous weapon will give you more total assault when you go into battle (it won't show up on the assault shown on the main screen; it's only calculated in battle). So it pays to upgrade SSRs first over SRs.
Lastly, you'll want to run the gemcha everyday. Save all Rs and use them to fodder your SRs, grails, and holy grails to raise the skill level of your SSRs. Each level of an SR is worth 3.5 skill levels when foddered and raising a weapon (i.e. a level 1 SR will give up to 3.5 levels to raise while a level 3 SR will give 10.5 levels to raise). Grails are worth 2 skill levels per level and Holy grails are worth 5 skill levels per level. So always raise your SRs to 4, grails to 3 and holy grails to 5 before raising your main SSR skill levels.
This should help for building your light team for now.
If you have other questions, we can help.
Dejnov.