Ext maths is soooooooo hard (basically its the workload of 2 mathematics courses). But.... since yr 11 doesnt count for anything, just do it and then when yr 12 hits, drop it or change to Mathematics as yr 12 mathematics is almost the exact same as yr 11 extension maths.
Physics and chem is hard as hell, i wouldnt suggest doing both because later on if you change you're mind about them, you only change one to senior science (Yr 10 Science) and the other one you can only drop.
For biology, once you begin the course (from day 1), you instantly get an assessment task which is like 30% or something. Its extreamly hard but its much easier (according to others) than physics and chem.
Advanced English is pretty much the same as Standard English although the only difference from i hear is that the text are different. In Advanced you will be studying old boring text i.e. shakespear etc and in standard you will study fun exciting easy modern texts. Although the syllabus i pratically the same. You still have to analyse, compare etc. Its just the texts thats different. But if you hate english so much then its still better to stick with advance as you will never "EVER" get a band 6 in standard(i know there have been only 1 person to get a band 6 in s.english, but c'mon that like winning the lotto). Also, if anyone is trying to convince you to go down to standard DON'T especially if your looking for a really high UAI (94 i think) as you wont get it especially if you choose to do the 3 science subjects.
Last but not least Software. Software is basically one of the most boring subjects ever. Most teachers who teach software wont actually teach you at all. Its better just to buy the HSC Excel text book and study that for 2 yrs (actually it takes only 2 weeks to study the full HSC courese of SDD, as basically all you do in class in bludge). Although SDD is easy for me, but for other people it may not be (probably cause its so boring, you day dream alot)
I would suggest you pick:
At least one science subject (physics would be better as it shows how everything is created etc but chemistry is fun when it comes to experiemnts, i love wasting and burning matches
)
Mathematics (pick extension if you want but be prepared)
Advanced English (and extension if you are REALLY!! good at it but most people fail so....)
Biology (hmmm maybe change (or you could change on your science sub.) to Economics and or business studies since you love commerce)
SD&D (Easy but boring + make you day dream alot + your surrounded by people who think they know everything, but in reality they don't)
Goodluck in deciding