AOG, it's pretty easy really.
You probably know what PMSL means (it is something TTT often does - I guess he keeps a lot of changes of underpants and trousers to hand at all times) so replace mysels with yourself and you get PYSL.
BMJ is British Medical Journal - you can look that up for yourself, though to be fair, this study was reported in The Lancet.
free-tee is TTT (three T but with the three pronounced in mockney - which is something TTT uses regularly in his posts since Cockney is almost dead these days having been replaced by Estuary or Sahf (South) London)
AS for the OP, the report on the BBCwebsite doesn't make it very clear, but the actual risk of a first episode of psychosis in the area of south London they studied is a fraction of one percent if I've done the sums right. Perhaps I am misinterpreting TTT's phrasing, but it does convey, to me at least, a degree of pleasure that people who use cannabis regularly are ata greater rish of pshchotic episodes than those who don't use it. Odd really, since that has been known for a long time, and this report just puts some numbers to the level of risk.