In 1.5.2 Beta WGT
How exactly do I get Soul Shards to work? Do I have to kill 64 to get to Tier 1 before it starts spawning mobs?
Because at Tier 0 It doesn't do anything and I cannot put it into a soul cage.
Precisely right, and for each tier you up it gains additional properties when in the soul cage, such as faster spawning, more tolerant of light, ability to spawn cross dimensional creatures (Ghasts in the overworld)
Something of note, if you say put in a 90 kill shard into a soul cage, you'll get back a 64 kill one when you break the shard, it rounds down the kills to the lowest that tier needs if you put it in a cage and then break it again.
Right clicking on a spawner with the shard of the same type will destroy the spawner and add 200 kills to the shard.
EDIT: Creatures spawned by a soul shard spawner won't count towards kills of another one, but mobs from a regular spawner count towards the kills too.
There is also a sort of cheaty method to get extra kills, look around, it's not hard to find on the forums, and I'm sure somebody else will happily tell you of it if you asked. But it's not how the mod is intended to be used, so I won't say it, even though it's worth mentioning.