you seem to be very new to riding bikes.. so my first advice to u is that unfortunatly like many other areas of life where we can lear from only reading or seeing bike is not something like that.. it not only needs practical experience but also expert supervision ...
so never ever go to murree road unless u are proficient in riding it .. it may take at least a couple of weeks .. so until then dont go there it is suicidal even for experts .. i have been on this road and it is quite dangrous and densly occupied...
now as far use of gear is concerned us the following technique
" only apply gear when u have pressed the clutch lever [in] ... i.e. clutch duba hoga tho gear lagavo.. AND second condition that needs to be fulfuliled before applying gear is that at the time of applying gear the race should not be applied .. in other words the u need to remove the race incase if u are going from first to 2nd or 2nd to 3rd and so on.. yani gear ma daltha waqt i.e jab app ka left pavon ka nicha jo lever ha us ko dabana sa pehla app ka race lever wapas apni free halath pa ana chahia.. free halath sa murad ka app na race ko gumaya nai ha... do not confuse the zeroing of race with zeroing the speed ..
jab gear lag jaya then quickl and immediatly race ko usi jaga tak la ka avo jider pehla tha and then phr aram aram sa thora thora race barhatha javo..
now as far other questions are concerned u need to know why gears are used... they are used to provide varoious required features .. i.e. agar charhai charna ho tho kam gear istimal karhta hai.. normally first gear tho majbori samaj lain ka start karka usi ma hi dalna ha.. 2nd gear ko or first ko use kartha hova app raja bazar ki sabzi mandi jaisa situations ma use karha hain.. 3rd gear is provides some balance between the 4th gear and 2nd .. i,e a good combinaiton of torque and speed.. and fourth gear is used for commuting on free roads ..
remember jitna gear high hoga utna hi bike ko control karna mshkl hoga.. kionka hi gear ma speed ziada hithi ha and bike ka inertia ziada hotha ha..
for 3rd question, bike ko app kisi b gear ma stop kar saktha ho,, but follow proper stopping procedure..
4th question: long and short lazimi nai but acha ha ka app 4th gear laga lo...
yar agar app na 30-40 pa chalana ha tho 4th gear is ok but 3rd is prefered blka app 3rd ma 30 tak hi rakain and then 40 sa ziada speeed ka lia 4th gear use karain..
only rexperience will tell u..
u should search the forum there was earlier also the same thrread that contains a good discussion and is a must read.. try u will find it..