The powers in primary and secondary sets come available at certain levels, whether you've taken the powers above them or not.
I'd suggest downloading Sherk Silver's CoH / CoV Character Builder to plan your build beforehand.
Irradiate does moderate damage with all the 'ticks' together, rather than per 'tick' It is also one of the best defence debuffs out there.
The slots should be distributed in the powers you want to enhance, but remember that more than 3 types of same enhancement in one power is a waste, so for example attacks are usually slotted 1/2 ACC, 3 DAM and 1/2 EndRdx/Rech.
You can later change your powers and slots (but not powersets) with 'respecs', which can be either earned by completing the respec trials, or getting awarded one by devs (so far they've been awarded when major changes to powersets have occurred, or in celebration of something, like the 1st CoH anniversary).
What Max said 
Irradiate does damage (in total) the same as x-ray beam. It also has a very good defence debuff (much better than others in the set) allowing you to hit more easily with your next attacks. It's a really good attack. Also, Cosmic Burst is excellent, not one you should leave out.
When you're slotting your attacks early on, it's best to stick with accuracy enhancements until you've got 3 or 4 in there.
IN MEMORIUM OF GAYBABY
CRUELY TAKEN FROM THIS WORLD WHILE SO YOUNG
Hi all, I'm new to the game and these boards, and after reading many posts hoped the community wouldn't mind me saying hello and asking for some Corruption advice
I've so far very much enjoyed the game, having not seen many games crafted as well or beautfully, nor with such a brilliant control system and community feel.
I liked the Corruptor character for his long range and support abilities. Though Energy looked like an interesting primary power set I opted for radiation as it has the defense penalty too, and Fire as 2nd for it's healing, and in three days have made it to lvl 12
So far I have:
Neutrino Bolt, XRay, Irradiate
Warmth, Cauterize
Swift, Hover, Teleport Friend
What I don't understand though is how some powers which I couldn't have chosen before were made available, despite not choosing the one above it. Is it possible to get any of the powers in the sets I chose regardless of which ones I've already chosen? I was ultimately thinking of something along the lines of:
Neutrino Bolt, XRay, Irradiate, Photon, Neutron, Atomic Blast
Warmth, Cauterise, Phoenix
All the fitness powers, Teleport and maybe hasten (which everyone on the boards seems to swear by)
I don't really know what to do with slots every odd lvl so have tended to distribute them a evenly except i have 4 on warmth. Is there anything I should aim for, or an order to try and follow to be a better team player/solo player? I know I would like teleport at 14 to make getting around easier, and phoenix at 16 if that is available, as ressurection seems to be useful. Any advice or experiences gratefully received. Hope to see you around in game
PS. What does irradiate actually do? It says moderate damage but only hurts for like 1, 1, 1, 1. Is there any way to change it to a better power?