Trust me topping is a piece of piss! look up uncle bens topping technique on here, really simple... What temp is your cab running at? leaves pointing up like that can usually be attributed to heat stress or environmental stress... and the yellowing leaves might be some sort of nute def if your not feeding 'em yet.
A good way to judge topping; If you look at your side branches coming off of the main stem, just chop above the 1st or second set to get 2 or 4 tops.
Ignore the full stops XD
(may go higher than 4 nodes but you can just clone the top if you have space)
....V <---- Main Stem/top
...--- <---- Cut here for 4 tops
...\|/ <---- Side Branches
...--- <---- Cut here for 2 tops
...\|/
....|
Don't know if that dodgy diagram is any use to you
As for nutes just get whatever you decide and read the bottle
got 1/4 - 1/2 strength to start then gradually build up to full strength.