If you do not have good farming mechanics, practice using bot games. Last hit with your Q on minions. Sheen can help you here if you have gotten Sheen. In the end, your Q will do OP damage which will help damage enemies. One Q to a champion can automatically have them running for their lives. To add insult to the injury, get lifesteal.

Pros:
Does OP damage late game
Tanky as shizz
His slow is one of the best slows in game
Spirit fire actually does damage when you have AP

Cons:
Got to farm a lot and last hit with Q
Can get shut down easily if not careful
Abuse of Spirit Fire can cause mana starvation
Can only focus max two champions

Always get your Q first. Your Q is your main source of damage and you need that to farm. Your Wither is second as you can slow your enemies when they try to run away. Last is the spirit fire which is there to help you farm and to damage enemies from afar. Your ultimate (Fury of the Sands) increases your attack range, health, and does area damage. This allows you to do more damage at a faster rate.

Armor and Magic resist is first priority. You can easily exchange Ninja Tabi for Mercury Treads if the enemy team is mainly AP. Boots of swiftness is a last resort if the enemy has hardcore CC. Sheen should be a good idea since it helps you last hit. Iceborn Gauntlet gives you the armor, AP, cooldown reduction, and mana. You can switch this out for a Frozen Heart if you want. Thornmail is for the armor and for those annoying melee AD guys who keep basic attacking you (serves them right). You can switch this out for a spirit visage if their is more AP enemies. Warmog's Armor is for the health and health regeneration. A Ravenous Hydra because of the extra lifesteal it gives you and because of its amazing ability to do OP damage. Last but not least, a Guardian's Angel for the revive and for the armor and magic resist.

If you have trouble farming, good idea to call someone and pull them out of lane to help you until you're good. Wither enemies that are running away from your team. Spirit fire in when you engage. Ignite enemies when you almost got the kill. Make sure you use help your team take hits and ultimate to give your team the advantage.

Overall, make sure you last hit with your Q. Alter you items if necessary to fit appropriate conditions. Help your team take hits. Thanks for reading my guide!