Litch is squashy to start so I find some armour is important to survive.
Mana. Yoou'll need loads. Don't cas a spell IF you can avoid it. You'll need items to restore mana, so heros, talents or items that do this are important (eg crystal maiden, consumables, shop )
Litch starts out fairly slow and you'll need to think about which boots really work. There is a big debate over this, some say tranquil, some say boots of travel, some say phase. Why not try the new boots of bearing
Given the mana issue, arcane boots are the best, but don't be afraid to experiment.
You'll need to shield the other player in lane, and use sinister gaze on opponents to stop them nuking you/the other players.

Consumables.
steady on the spamming of consumables. Litch will be a slow drag if you spend too much on these.

Litch as a support needs to place wards. The great thing with Litch is when pulling a lane( learn how to !) you can shield the tower, pull, and then watch as the mega wave comes down and realy hammer the lane.

Chain frost is a great jungling tool

So you'll need to think about spell cast distance and force.
In the lucky dip section are items that may come in useful. or not

Litch can really become a tank , but you'll need to work on cast speed, walkin/movement speed.

Litch is really magic reliant, so becareful. Up against jugganaut, you'll have to think carefull as jugganaut is magic resistant.
Litch has NO escape, and is liable to be blonked, so be careful.

Litch can work really well with team members who can stun or trap, as frost blast, sinister gaze and chain frost will hurt enemies.

With Shadow Shaman, Natures prophet or witch doctor on the team, Litch will really be useful. Magnus too.

Against players who use blink, or invisibility, Litch has problems and becomes Itch. Choose wisely!

Stay back if you can, dont farm alone, dont let the tower lull you into a sense of false security