Xerath is my favorite champ right now when it comes to just playing a fun champion that totally and utterly destroys his lane. Xerath is a very strong mid champ because of his giant range and his very consistent burst damage. Xerath doesn't take much skill to play once you get used to timing his skills e, r, r, r, q or e, q, r, r, r, q if you can hit 4-6 of these skills you will insta burst almost anyone you are facing mid.

Pros:

Outrageous range longest in the game by far
Crazy damage in the lane can out damage many champs
Has a reliable stun once you hit 6

Cons:

Falls off a little bit late game
Can't compete with alot of other mages cc

I take Magic Penetration Greater Mark of Magic Penetration Marks , Manna regeneration Greater Seal of Replenishment Seals, Ability Power per level Glyphs Greater Glyph of Scaling Ability Power Glyphs, and Flat Ability Power Greater Quintessence of Ability Power Quintessences. I take all of these runes because it allows me to stay in lane for a substantial amount of time with out manna issues it also allows me to do a great deal of damage at 6 which is when I start to really just harass the **** out of any champion, also this will make you alot stronger in the later game too.

I take a 21/0/9 mastery build maxing in all damage in offence and manna regen and neutral buff duration in Utility. I do this for the similar reason as the rune set up to stay in lane longer and max out damage for the game.

I start out with one Doran's Ring or Boots this is depending on which champion I will be facing mid some champions to look out for early are Sion, Brand, as well as Morgana all three of these champions are very annoying to lane against and if you start with boots this just allows you to dodge skill shots or avoid harass easier. After your first recall you should be able to grab another 2 Doran's Ring or grab your Sorcerer's Shoes. After your first recall you should already be about level 6 and then it is time for kills. Next after I have my core out which is simply 3 Doran's Ring and my Sorcerer's Shoes I begin to start buying pieces towards my Rabadon's Deathcap starting with the Needlessly Large Rod as soon as you get your Needlessly Large Rod you can start ganking other lanes I try and gank bottom with the jungler after sending mid back and pushing the lane. If you gank bottom successfully you should be able to grab your Blasting Wand or have pretty close to it after selling a Doran's Ring. If your team is doing well at this point try to team push mid and start a fight if not keep gankinng the other lanes. Your final build should look similar to the one above because your range is so giant you don't need much survivability.

I start with Arcanopulse max it first. Grab MAGE CHAINS at level 2 and max it second grab Arcane Barrage whenever I have the chance level 6, 11 and 16 and get LOCUS OF POWER at level 4 and max it last. This skill sequence allows you to do the most damage and harass in lane and as soon as you leave and get into team fights you can do ridiculous aoe damage.

I take Ghost and Flash as my summoners Xerath doesn't have a escape at all so it is always nice to have both of these summoners.
Some other summoners to think about.
Ignite: Great mage spell I just don't really use it on xerath at all
Exhaust: Not bad allows you to win 1 on 1 fights and add a bit of extra cc

It is very simple to farm as xerath you can easily last hit with auto attacks because of his great auto attack animation. Once you get 2 or 3 dorans you can even just last hit with your q it doesn't insta kill minion waves but it does alot of damage and you cna spam it to clear waves I would not advize you doing this unless you want to push your lanbe to kill a turret or just clear there minions away from your turret.

In the laning phase you can easily farm and harass against anyone because you have such a giant range. You have outstanding damage and a stun. If the jungler ganks you should get a ghost or flash out of the enemy or a kill every time. After you push down your turret or even send someone back whether they b'd or just got killed by you. I usually gank bottom or top which ever lane is losing or being pushed and just rape there faces. After the laning phase is over you should be able to not get focused in a battle because of your massive range and you can stun anyone who comes at you. Late game always focus the carry with your e r r r q combo and you should win every team fight.

Always ward if you are winning the lane if not ask your jungler to help you out by warding. Always help with team objectives because your mid if you are getting dragon or baron go help you may be needed and the experience it gives you is pretty sweet so just help with team objectives as often as you can.

Xerath is a beast champ he can carry like a boss and you easily get fed enough to pull your team out of elo hell.