So, I am not a pro, or even ranked. I just play a lot of Pantheon and I have for years.
You're ok. But there are some obvious things you're missing. Some missed cs that really should not have been missed, a few misclicks on abilities. Your first kill against Nasus was really more about the Nasus being bad than you being good.
So about your starting items, the usual Panth start is the Corrupting Potion because as you saw, if you keep chucking your spears you run out of mana quicker than you can imagine. Doran's is also fine, but it's a bit riskier especially if your against a champion with sustain. If Nasus had pulled back and used his abilities to heal and his own potions you would have been out of luck to really pull off your full level 3 burst. So, if you're going with the Doran's start you need to manage your abilities and get ready for that full burst. And it'd be great if you think a step or so ahead, if you don't push the enemy into their own tower before level 3 you can generally get the kill. Now, you can be a bit more aggressive with the Nasus, because you should be able to aggressively counter them in lane.
The potential mana problems come up again with the kled counter-gank. One more spear and you would have gotten the kill.
Alright, that out of the way let's go to build order. First off, Pickaxe. Not a bad first item just by stats, but not all that good by build paths. You got an early Last Whisper, but look at what that gives you against most opponents at the time you got it. If your opponent buys a 50 armor item you lower it by 15, which is about the best you could hope for. That's good on exactly one opponent, the enemy tank, and you're post level 6. Your ult and teleport, that means you should be going around the map and wrecking shit up.
I'd instead suggest looking at Ghostblade. Serrated Dirk is cheaper than Last Whisper, the 10 ArP is comparable to LW 15, and would be actually effective against non-tanks. Plus, to maximize its effectiveness you need to throw spears between last hitting. Which is what you want to do. It forces you to focus on last hitting, which is the big key to getting better.
The other generally considered key item is Black Cleaver, health, damage, shreds total armor (useful all game long), and increased move speed lets you stick onto your opponent.
Next up: WARD. WARD. DROP MORE WARDS. WHY ARENT YOU WARDING? Seriously though, if Rengar had been a bit more aggressive top post 6 you would have been screwed with your ward placement. You got lucky, that he was down bot for most of the game, and, his one big gank was just handled poorly.
Most of the teamfights were ok. You got greedy a fair few times and essentially suicided. This is mostly stuff you'll pick up on instinct after awhile.
So, that's mostly it. You're ok, but there's room for improvement. Out of curiosity, what opponents do you lane against that give you trouble? And, knowing that Panth's effectiveness falls off late game, what steps do you take to make sure the game ends quickly?
and 
. Last whisperer only shreds BONUS armor which nobody will have much of in the early game. You should have gone 
anyway because they really dont have a lot of self healing. LW items are like a third item at the very earliest. They are extraordinarily inefficient as first items. Maw is also a questionable choice considering the only major source of AP damage is Donger. Nasus does do some Ap damage, but its still mostly AD from his Q