Not sure at what point you're at right now, so what i might tell you is completely unnecessary, but helpings fun, so whatever.
On the topic of Skeletron:
MOLTEN.
ARMOR.
Skeletron hits like a bus in Expert mode.
Yes, that means saving him for last in Pre-hardmode. You gotta kill EoW for the Demonite to mine Hellstone, but trust me, it's worth the effort.
Dealing with him
Some things recommended here are mentioned later, so sorry about that.
Skele
takes hits like a tank, as well. At least, until you destroy both hands. In Expert mode Skeletron takes VERY little damage to the head, in the 1's and 2's type of little damage. You HAVE to take out the hands before going for the head.
You can take out the hands somewhat fairly using Spiked Balls, from the goblin invasion. But those won't do themselves. I recommend Meteor Shots and a Minishark to take out the hands, because since they take no knockback anyway, they won't be an issue for the shots. I recommend at least 2 stacks. (1998)
If you have Spiked Balls, you should have done the goblin invasion. Get some Rocket Boots from the Goblin Tinker, combine them with your Hermes boots. The elevation with them should work well when dodging his slaps.
Once his hands are down, Keep him at range. He'll be firing skulls at this point, so stay mobile, and keep firing at him. MELEE WHILE HE'S SPINNING CAN BE RISKY! He does a LOT of damage, even with Molten Armor!
Potions and Buffs
As for potions, try using the following:
Ironskin, Regeneration, Thorns, Mana Regen/Power (
If you use Mage items), and Swiftness. And if you're the fishing type, Ammo Reservation. Making it requires a Double Cod, caught on the surface (or sky) Jungle.
Unless you're some sort of dodging
GOD, you'll need thorns. You're bound to get hit at some point, and with the damage he does, the damage reflected should be good.
Buffs, you'll want a Campfire, for sure. Easy to make, and the passive regen, although small, might save your life.
If you have a spare heart crystal, make a Heart Lantern.
Again, if you're a mage, a Star in a Bottle is a good choice too. Both the Lantern and the Bottle give very useful regen effects.
Accessories
At this point, you should have the Worm Scarf. That is a Must. It reduces all incoming damage by 17%, which can go a long way!
As said before, the Spectre Boots are a real boon here. Getting some quick elevation could be very useful. A substitute for that would be the Fart in a Jar, or Blizzard in a Bottle. The regular Cloud in a Bottle doesn't give as much height as those, so it is recommended to get them instead, or even the Balloon variant for extra height.
As for anything else, i'm not sure. I'm not too much of an expert on the Accessory loadout for things, but the things i mentioned are musts.
Also, it's a good idea to get a lot of Warding or general defense modifiers. Although that'll cost a bit of coin in reforging, unless you're lucky.
Viable Weapons
The Minishark: A Must. You simply cannot kill the hands as easy with anything else. The Rapid fire rate and damage, along with the range capabilities
The Fiery Greatsword: If you want to be a bit risky, the FGS
I Like to say Figs is a very good choice. The great damage output is really useful, and it has fair range. But like i said, Melee is risky.
The Imp Staff: Although it may not seem life saving, having something still damaging Skeletron while you're busy dodging can be a huge bonus. However, the imp might target the head, and if you haven't taken the hands out yet, that will be worthless, since all he takes is 1 Damage, a whopping 2 if it's a crit.
The Molten Fury + Hellfire Arrows: The Molten Fury bow can do a good amount of damage with the Archery Potion. Hellfire Arrows deal good damage themselves, and Explode on impact. The explosion doesn't damage you, and does good AoE, so you could damage both hands at once if you're a good shot!
The Arena
The arena is a very important factor.
You're going to want a Flat, wide, multi-level Platform arena. Try different patterns, like moving in a circle, an 8, etc. Experiment, i'm sure you'll find something that works, because i sure haven't.
Having a lot of hills and pits is just going to kill you. Fill things in, level hills, and make those platforms!
Whew, and that's just skeletron prep...
It would be hilarious if you already beat him, though. 
I Really hope you didn't so this would be useful, i spent like, and hour on this.
