I need help with Agni

Can some explain Agni to me? I suck with him and well I'm hoping to improve also how should I build him?

Let me preface this by saying that Agni is not easy to play. While not particularly mechanically difficult (aside from one thing I can think of, more on that later), his kit has a lot of depth and application that is really best learned through experience. That being said, here's a short summary of ways to improve:

Firstly lets talk builds. Agni is a fairly straightforward character, you need a lot of damage, you need to be able to burst people down with your ulti combo, and you want to be able to do this as often as possible. To do this, you need a build with a lot of power, penetration and cooldowns. My recommended build is:

Book of Thoth / Doom Orb ( Book of Thoth is more reliable, gives you near infinite mana and has a better late game, Doom Orb has better early kill potential)
Shoes of the Magi / Shoes of Focus (early damage vs more CDR, either is viable)
Chronos' Pendant (you can swap the order of this and your fourth item, but I like grabbing it third)
Spear of the Magus / Obsidian Shard (only use obsidian shard vs fairly tanky teams, Agni has a lot of chained combos and tick damage)
Rod of Tahuti (Duh.)
Soul Reaver / Spear of Desolation (pick one, reaver has more burst but it's less reliable (but does synergize with Book of Thoth with its mana bonus), desolation is super expensive, but can let you completely wipe enemy teams with its passive).

You can also swap out your 6th item for a lifesteal item like Bancroft's Talon, but generally I find that this isn't desirable.

Moving on to mechanics! Agni is a very complicated god when it comes to this but I think I'll just give a few tips on his necessities.

1. Skill order: Your priority is Rain Fire -> Flame Wave -> Path of Flames -> Noxious Fumes. However, level 1 you should level your Path of Flames since it has more base damage with all of its ticks. This makes clearing red buff easier, and the first one or two waves can be cleared by dashing through the wave (start at the back archers, end at the melee attackers). Do NOT continue to do this past the first couple of waves, you're burning your only escape every time you do so, this is simply a tool to help your early clear.
2. Ultimate combo: Your basic combo is Rain Fire -> Noxious Fumes -> Rain Fire x2. This is a true combo on gods without leaps, you'll know you're doing it correctly if you stun the enemy god on the first Rain Fire since the Noxious Fumes should come out before the shot lands. To accomplish this I recommend quick cast since it makes the timing easier than clicking back and forth between two abilities on default cast. Remember! You're always a threat since the range of your Path of Flames -> Rain Fire is extremely far.
3. Using Noxious Fumes effectively: Noxious Fumes is your best friend. Its stun is really the core of your kit, here's some ways to use it that I think are underlooked.
   • Noxious Fumes -> Path of Flames -> Flame Wave. If you place down your fumes and then immediately dash through them, enemies caught in it will be stunned inside your path, getting the full brunt of the tick damage. This can do a MASSIVE amount of damage, especially when paired with your Flame Wave, just take into account that you've burned your only CC and escape upon doing so and are now completely vulnerable.
   • Path of Flames -> Noxious Fumes. If you're in a bad spot, the best way to get out is to dash and then turn around and put down a Noxious Fumes on top of the burning path. This will stun any enemies near your path and allow you to widen the gap between you and pursuing foes.
   • Flame Wave -> Noxious Fumes. Burnt your Path of Flames already? If you're still in a pinch, Flame Wave can actually activate a Noxious Fumes if it's cast immediately afterwards near the end of the wave. This can grant you a super quick stun, either for engaging an enemy or for running away when your dash is down. Do note that this is mechanically difficult to pull off, even with quick cast on.
   Overall though, just avoid using your Noxious Fumes before casting any ability (except for dashing through with Path of Flames). Enemies can simply walk out of it before you activate it with any ability aside from your dash.

That's all I can think of for now, good luck doing Swagni things!

Only reason to put down fumes is if the enemy is already CCed so you can put in some extra damage and prep a chain stun. However this is only for short CC as otherwise you need to AA which doesn't do a lot of damage (unless you want your passive up)

Never underestimate the damage your passive can do. I have killed lots of escaping enemies with it.