Most urf games will allow you to partake in the obligatory ARAM phase at top lane, after which every player goes to their respective lanes. You could start a longsword and E ability to reach targets and lock them down, but ideally, you want to start with your R ability, an Amplifying Tome, and try to snag some kills.

During laning phase, you want to rush either Lich Bane or Nashor's, and try to build into Alternator and basic boots. Having these 2/3 items will give you enough burst to take down most opponents, and the basic boots will allow you to use predator for extra burst damage or to gank other lanes (which you should be trying to do a lot). For a more in depth explanation on itemization, look at the items portion of this guide.

As you acquire the essential Nashor's Tooth, Lich Bane, and Night Harvester, your damage will spike greatly. Try to rush Rabadon's Deathcap and finish your boots at this point. You should be looking for even fights or advantageous, or rarely very slightly unfavorable fights in the enemy's favor. This build will not be consistently great against many targets at once (alone) until you are ahead or until you have Deathcap. Do not let this deter you too much from team-fights, as your AOE is an insanely good damage output asset in those situations.

At full build and with an elixir of sorcery, you should be deleting squishies and dealing significant damage to tanks (rageblade is a good item to build into if you are facing a very tanky team). Mow down anything in your path with Udyr's spirit fire and pave the path to victory.