which support gem should i use for spark?

hi i'm just beginner, this is my first playing trough of poe.
i am playing a lightning sorc. at the moment i use for spark: faster casting and fork gem. it's not bad, but are there better combinations with spark?

You've kinda got the go-to supports. The only other ones that usually get used early game with it would be reduced mana cost or spell totem.

Spell totem is nice for conserving mana and utility. However, will reduce both damage output another 40% and cast speed by 30%. The latter can be addressed with high level Faster Casting.

Reduced mana cost avoids both penalties while still conserving mana, but lacks the utility of being able to drop it through a doorway or across chasms.

Greater Multiple Projectiles and Lesser Multiple Projectiles are not quite as useful on spark because it already fires 3 sparks, but if you've got spell totem and skill sockets to spare GMP is a lot of fun with fork. If not using spell totem, just avoid these on spark.

Other supports it'll synergize with is lightning penetration, elemental proliferation, increased duration, and faster projectiles. The last 2 will work together to make sure you clear all corners of the room and the adjacent ones as well. Lightning penetration will just help deal damage overall. Elemental proliferation will help make sure everything becomes shocked (which will increase damage dealt from 40% to 220% depending on 1-3 stacks of shock).

EDIT: Silly me, I forgot Life Leech as well.

big thanks to you :)
i have tested chain gem + spark a few minutes ago. the dmg is nice, but the mana costs are incredible...even with clarity i'm not able to manage thehigh mana costs.

I'm surprised you have your hands on a chain gem!
If you end up stacking a lot of high mana cost modifiers to spark, that's where the spell totems come in handy. For a large mana down payment the spell totem will continue to cast as long as it's active (base 15s which is 16 casts after penalty without faster casting).

EDIT: P.S. It will also work with pierce and mana leech.

I link mine with fork, mana leech, and faster casting.