I'm not a ranked player; I've played two games there and lost both of them(Neither of them were as Pantheon, though). Mostly, I just play normal blind picks, and I usually play Pantheon. So while I'm no high ELO hero, I've gotten fairly decent at stomping lanes in normals. With that said:
Panth isn't your standard AD champion. Champions like Trynd, Yi, and Fiora are melee AD carries; the bulk of their damage is in their autoattacks. Pantheon is an AD caster; the bulk of his damage is his abilities, specifically his Spear Shot and Heartseeker Strike. Think of him like a mage with a spear and you should start improving. Attack speed and crit are much less important to him than raw AD.
For runes, I like flat AD reds and quints, scaling mana regen yellows, and mres blues(flat or scaling). The flat AD lets you last hit better, and the armor and mres help your sustain. You can switch out for flat armor yellows if you want the added durability. I'd run armors in higher tier play, but for blind normals, I find that mana regen lets me be so aggressive I can usually stomp the lane easily. Against more skilled people, I'd probably want the armor.
Masteries are a standard AD 21/9/0, and for summoner spells I like flash and either ignite or exhaust. If my team has no exhaust, I take exhaust. Otherwise, usually ignite.
Skillwise, I generally do this: QWQEQ. After this, max the spear first and heartseeker second. I usually put off my second level of jump until L13, maxing heartseeker at 12. The jump is, I find, mostly a positioning thing since most people won't stand in it. Since that's the case, I prefer getting my max heartseeker a level early. After the first point in the stun at L2, ignore it till you have nowhere else to put points.
Itemwise, start boots and 3 pots or cloth armor and 5 pots. I prefer boots/3 the vast majority of the time, only taking cloth/5 when I know it's gonna be a rough lane. My next item is almost always a Brutalizer, for the cheap early AD/ArP/CDR. If you have to go back before you get a Brutalize, pick up a Doran's Blade. You can always resell it. After that, a go for a Bloodthirster; since Panth's an AD caster, this is by far the best of the BF items for him, giving him needed sustain and a ton of AD. For boots, I go Mercury's or Ninja Tabi, depending on whether or not I need the tenacity, or the armor/BA reduction. After the Bloodthirster, I usually go for a tanking item. Frozen Mallet for health, Guardian Angel for armor/mres.
Of course, try to buy a few wards everytime you go back in addition to your other things.
A situational item for me is the Hexdrinker, which I'm becoming more and more fond of. Gives some great survivability against AP, and now it upgrades into the Maw. If I'm laning against an AP, this often replaces the Brutalizer. Another situational item for me is Wriggles. 1600g for AD, armor, a free ward, lifesteal, and a powerful farming proc that helps take objectives is pretty solid. If I take this, though, I'm probably going 1v2 and started cloth/5.
Playstyle is pretty aggressive. To start off, autoattack 4 times to get your passive up. You always want this passive up. It's awesome. After it's up, start last-hitting while harassing with the spear. Don't go overboard at low levels, though; you want to be able to start going for a kill at L3-4 when you've got your stun and heartseeker, and you can't do that if you're tapped out from L1-2 harassment. If you've got to, use the spear to last hit, but try to do so sparingly.
You've got a lot of early game burst; your spear should be keeping your opponent cautious. If they're at 50% or lower, usually, a spear/stun/heartseeker/spear + ignite will often kill them. If not, it'll probably drive them back. Abuse your passive to towerdive if they're low, but be careful. Going 1 for 1 is often worse than just letting them recall while you keep gaining exp/gold in the lane.
While laning, keep the pressure on. Panth's an aggressive champ who falls off late game, so you want to dominate early. If you kill your lane opponent, think about ganking another lane. Panth's a powerful ganker, especially after 6 when he gets his jump.
I'm kind of writing a book here, so I'm gonna cut myself off here and leave off with this: the key to playing Pantheon is learning how to play aggressively without playing stupidly. It takes some practice, but it's a helluva lot of fun.