Starting items:

Second round:

Third round:

Start of with a Regrowth Pendant and a Mana Potion. This will keep you in the lane and allow you to harass with your spells. If things don't go too well, just save up for Philosopher's Stone and ideally Boots of Speed aswell and recall. With your already great farming skills, the health and mana regen should make you unkillable. Ideally if you come back for the second round, you will have a BF Sword or atleast a Vampire Scepter. If you are not in mid, getting a BF Sword might be too much. During the third round we should have farmed enough to have our core build together. Boots of Swiftness, Bloodthirster and your Philosopher's Stone.

As of then it's mainly depending on how the game is developing what your next item should be. If they have many AP based champions, buy a Banshees Veil. If their fighters are doing very well get a Chain Vest, we can later transform into a Thornmail. Unless the tank can't handle the damages you should be in the back and more damage sounds like the better way to go over more armor at this point.

Mid/late game:

Late game is approaching and we should now be dealing a nice amount of damage during teamfights. If you notice you are being outrun when chasing surviving enemies, you might want to get Frozen Mallet. If not, my next item mostly depends on whether the enemy has a high HP pool or not. If they do, I get my Madred's Bloodrazor over my second Bloodthirster. It's passive is just too wonderful. The attack speed it grants is also a very nice addition. After that I either finish my Thornmail/Banshees Veil or get another Bloodthirster, depending on the damage I am taking.