Lux is a begginer-friendly damage oriented mage that can be played in the mid lane and Support role.
This guide is about playing her in the support role.

So basically you passive Illumination makes it so that if you use an ability on an enemy, your next AA on them will deal ability power, it is good to finish an enemy.

Your Q, Light Binding, is a skillshot that binds the two first targets hit. Hold your Q so that you have a way to hold down the enemy if they engage.

Your W, Prismatic Barrier is a shield, max it last but it's still important, it can save you from death by an enemy's Ignite.

Your main spell to do damage and poke is your E, Lucent Singularity, you can almost always spam it in lane to annoy and poke your enemy laners.

Your ult, Final Spark, does a lot of damage but it's actually easy to avoid, personnally, never ult if the enemy can move, always Light Binding first.

In Lux support your primary task is to poke the enemy laners and to deal a lot of damage to the enemy team. Lux is a support that likes to take kills but then the ADC becomes useless, so usually I try tu share the kills with my ADC if he can't take the kills in the 2v2, I will not try and it is not recommended to steal your ADC's kills. It is easy to take kills by accident because of the insane amount of damage you can deal with Lux.

Lux in not so much of a roaming support but I recommend roaming to mid and using your Q on the enemy mid for an easy assist and a kill for your team. Do not hesitate to help your jungler if he is getting invaded and loses the 1v1.

If you are being camped by the enemy jungler and your jungler doesn't come, back up with your ADC and try to kill the enemy jungler while the enemy ADC and Support are farming. Or you can just set up a good situation with your mid laner in the river to stop them from getting the dragon.

Before level 6,
Light Binding --> Lucent Singularity --> Lucent Singularity --> AA

After level 6,
Light Binding --> Lucent Singularity --> Final Spark --> Lucent Singularity --> AA
It's a garanteed one-shot on squishy champions if you aren't too behind.

So your role in teamfights in in the backlines, be protected by your tanks and deal a lot of damage. If you have a hypercarry, stun the enemies with Light Binding for them and shield them with your Prismatic Barrier. If the enemy has a hypercarry, Exhaust them and bind them.

In the late game, you can even start teamfights by using your Light Binding on them and killing them with your combo, if the enemies then jump on you, your team that was waiting in a bush can surprise them and that'S an easy teamfight.

It is very easy with Lux if you are a bit fed, to pick off somebody and just one shot them with your combo if you land Light Binding. You can even do that to enemy carries if they are squishy but do a lot of damage.

If you are behind, try to participate in as many teamfights as possible, even if it is just by using your Light Binding to immobilize the enemy team or by using your Prismatic Barrier on your teammates. If it goes well, the enemy could lose their lead.

Even if you are fed as Lux, you can always get one-shot by enemy 0/5 Shaco, Irelia, Yasuo, or Yone because of Riot Games character design, so look be wary of your surroundings.

Look out for abilities that can negate your abilities like Yasuo's Wind Wall or Samira's Blade Whirl.

In teamfights, try to hit as much people as possible at the same time with your Lucent Singularity and Final Spark. Don't forget to shield your teammates with Prismatic Barrier.

So there are a few things that you need to know about Lux other than the ones already said here.

Your Light Binding is a great tool to stop recalls with the good range and you can follow up with an Lucent Singularity.

Your Lucent Singularity is a very good tool to tower dive or poke the enemy when he is under turret.

Your Final Spark is a very very good objective stealer, I have stolen Baron or Dragon many times with it and it is fun to see the enemy team so surprised when a Lux ult comes from nowhere to steal the objective. Your Final Spark is also a very good way to kill the recalling enemy because your passive gives you vision of them for a few seconds, so just wait a bit so that they have opened the shop and then free kill.