Gems are a fundamental part of your build. Getting the correct link setups before entering Maps and leveling those Gems as you progress through the Atlas is one of your main goals. This is how your Gem setup should look like before entering Maps:
Explosive Arrow Setup
Explosive Arrow supported by Ballista Totem Support is our main damage dealer. We add Faster Attacks Support to apply fuses faster and Ignite Proliferation Support to improve clearspeed.
Once you have a fifth link, add Deadly Ailments Support and as a 6th link add Elemental Damage with Attacks Support.
Once you are at endgame and you have Divine Orbs to buy better gear, replace Deadly Ailments Support with the better Awakened Deadly Ailments Support and Elemental Damage with Attacks Support with Awakened Burning Damage Support.
Arcanist Brand Setup
Arcanist Brand triggers supported skills on enemies it is attached to.
One of the skills you trigger is Flame Surge, which applies a Burning Ground that deals damage to enemies standing on it.
The other skill is your main curse: Flammability, which reduces enemy Fire Resistances, which is especially important against Bosses and tanky rares.
Your 3 main Auras are used to increase your defensive capabilities.
First, Determination grants you a huge amount of Armour, which reduces incoming Physical Damage significantly. On top of that, you use Grace which grants a chance to evade enemy attacks outright.
These Auras are supplemented by Defiance Banner which increases both Armour and Evasion further. Don't think too much about putting your banner down, it's mostly there for the passive bonus it gives.
On top of the defensive Auras, you also have Precision, which increases your Accuracy substantially. This solves one of your weaknesses, since you are a Bow Build starting in the Witch area.
The amount of Reservation Precision takes depends on its level, so always check if you can support leveling it up before!
Lightning Spire Traps only purpose is hitting enemies to apply the Combustion Support debuff. Because of Ancestral Bond, you don't deal damage yourself, therefore you are unable to Ignite an enemy and apply Combustion Support. To circumvent that, use a Trap because it still deals damage.
Use Flame Dash as your main Movement Skill. No Support Gems are needed.
Whenever you hit an enemy with Frenzy, you gain a Frenzy Charge, which gives you more Damage and increased Attack Speed. To refresh them during boss fights you need to land an attack periodically. Faster Attacks Support is only there to make the skill feel smoother.
Vaal Molten Shell
Molten Shell gives you a defensive shield that can absorb enemy damage depending on how much Armour you have. Since Molten Shell has no cast-time, put it on your left-mouse button(move) to automatically activate it whenever the cooldown is up. Vaal Molten Shell is something you only press whenever you are in danger. It makes you almost invincible whenever it is active.
Summon Lightning Golem increases your Attack Speed slightly. It's not important for the build but it's better than nothing. If you are starved for sockets, this is the gem to cut first.