Ok lots of answers to your questions can be found here: http://www.totallywicked-eliquid.com/forum/mixing-discussions/mixing-101-11504/
If you want lots of vapour, go for a higher VG/AG mix, if you want higher throat hit, then you need more PG in your mix.
Most premixed liquids are around 80%PG to 20%VG.
When ordering a kit I would advise to order either PG or VG, and then make a separate order for a bottle of the other.
This way you can play around with your mixes, and decide what you like best.
Concentrates are normally PG base, so you need to take into account what % of your mix will be concentrate, what % will
be nic base, and then figure out how much of the other AG/PG you will need. Sometimes it can be both.
The difference between PI and TI... PI is 54mg nic, TI is 72mg nic. So if you are mixing 18mg for instance, from PI you would
get 3 times the amount, from TI would get 4 times the amount.
Hope this helps a little, I may have just confused you more