What items should I buy at the start? And in mid game, what items should I aim towards?

Courier/wards
2x minor totems
2x runes of blight
2x mana pots

mid game aim towards boots (ghost marchers if you can afford it), power supply, bracers and more wards.

This user seems to know what he's talking about, vouch.

You're mainly a support, your job is to buy the courier/upg it and ward/counterward. As he said, ghost marchers whenever you can afford them(remember keeping wards up IS your priority). Good ganker too, around mid-game you should be ganking lanes with your coktail-cursed ground. Ulti in team fights, buy staff of the master if the game drags very, very long.

I'd prefer level 4 codex, it guarantees a kill after cursed ground.. In public games I just buy boots and then instant codex to frag lol.

In pub games (which is all I play) I just buy Ghost marchers and then go right for staff of the master because it splits your ulti which is fucking rape. After that I go shroud this way I can stun, curse ulti and go invis so they can't interrupted my ulti while it's raping in team fights and shit.
Also used to bounce out of fucked up situations.