I cannot stress enough that skillbuilds are FLUID. Do not mindlessly take over this build,
try and adapt it to different situations that might arise ingame.
1 Frost Nova / Dark ritual / Frost Armor
2 Dark ritual / Frost Nova
3 Dark ritual / Frost Nova
4 Frost Nova / Dark ritual
5 Frost Nova
6 Chain Frost
7 Frost Nova
8 Dark ritual
9 Dark ritual
10 Frost Armor
11 Chain Frost
12 Frost Armor
13 Frost Armor
14 Frost Armor
15 Stats
16 Chain Frost
17 - 25 Fill 'er up with stats.
Start with?
Frost Armor
Situationally, you may get Frost Armor at level 1 if your team is attempting a level 1 Roshan. The extra armor and melee slow will help tank (but make sure, you genius, that it isnt YOU who's tanking Rosh).
Dark ritual
In my humble opinion, getting Dark Ritual at level 1 is mildly retarded. A measly free deny or two AoE slow nukes (your manapool allows two right off the bat) for gar frosty lvl1 secks? Gee, lemme think about that durr.
Nova or Ritual?
Burst versus spam. If you're laning with a partner that makes the most out of the slow (eg. Riki, Void, PA, Vagina) get an extra level of ritual. If your lane relies more on burst damage (eg. slayer) get more Nova. If you're laning with neither (eg. Dirge or so lol) then any will do, whatever.
Chain Frost
Your ultimate is gamebreaking and has a reasonable manacost. Get it ASAP. There is no reason to delay it.
Maxing your skills
Maxing Nova
Frost Nova is your bread-and-butter nuke and should therefore be maxed first.
Maxing Ritual
Frost Armor doesn't reach its peak effectiveness until late-game. Maxing Dark Ritual next will give you very effective mana replenishments in between ganks.
Stats
Do not get stats before maxing out all your skills. They all do a fixed amount of damage and their burst damage effectiveness wanes late-game.