I am an amateur player with the dream of becoming pro. I have quite a lot of guides and would recommend you check my guides out.

Pros
Good AoE damage
Good resistances
Good sustain
Incredible single target damage
Strong pusher due to her passive

Cons
Needs farm
Fairly mana hungry early game

Take VS
Katarina
Anivia
Twisted Fate
Karthus
Ahri
Orianna

Don't Take VS
Mordekaiser
Swain
Ryze
Galio
Cho'Gath

In lane, you want to just farm early on. Farm on Diana is crucial. Until you buy your Haunting Guise. It isn't worth going for kills, unless your jungler ganks. Diana has a lot of damage if farmed. It is often worth putting your W on when going to farm minions as the majority of the time, you will be against a ranged champion. When level 6, as Diana the combo is Q-R+W-E-R If you think you have enough time you can go Q-R+W-E-Q-R meaning you keep your ultimate.

Tip #1 It is worth asking your jungler if you can get blue as much as you can. This is because although your Chalice of Harmony gives a good amount of mana regen. It isn't as strong as blue buff. Tip #2 If you think the enemy jungler is about to gank, and waiting in the bush, you can Q the bush from a safe distance allowing you to reveal the bush without being in danger. Tip #3 From level 6, always take Wraith for extra gold, just Q and R onto them and then use your W to take them out. This is often good when your lane is pushed as you still get gold, without putting yourself at risk.

Thankyou for reading my guide, I hope it helped you and you were/are going to be successful with this guide, it works for me. Please feel free to check my other guides, peace out, JiJaSim.