motor cycles have a constant mesh gear box...
you need the clutch to get moving in first gear( slip the clutch as you would on a car).. IE. 3000 rpm let the clutch out till it starts to grab.. bike starts moving let clutch out more until all the way out..
now your mobile.. & the clutch is used to help change gears (but you do not have to use it)
Try this.. it will give you an idea how much the engine slows down as you change up gears..
riding in first or second gear.. with about 4,000 RPM on the tacho..
put a small amount of weight on the gear lever (as if your changing up a gear). now roll off the throttle.. the bike will change up the gear without the clutch.. what this means is that as you change up gears the motor needs to loose revs to maintain the same speed.. once you know how much the motor slows as you change it will become second nature for you to allow the motor to slow as you change up.. giving a smooth up shift.. then you accelerate in that gear.. roll of as you change up.. smooth..(practise makes perfect)
conversly.. as you change down you need to increase the revs as you change down gears.. try giving the throttle a quick blip..as you pull the clutch in & change down the gear.. practise will tell you how much you need to blip..
You can change down gears with out the clutch, but its harder to do smoothly & has no real benefit.. If you want to try.. ride in 3rd or 4th gear at about 3000 RMP.. put the small amount of weight on the gear lever (as if you were changing gear).. now accelerate & the bike will change down the gear.. (this makes the bike accelerate quicker) so be carefull doing this...
slipping the clutch is required when taking off in first... all the other changes should be made as quickly as possible.. (if the clutch is in theres no drive to the rear wheel.. therefore less control)
braking & down shifting should happen at the same time.. you should always be in the right gear for the speed your doing.. just in case you need to accelerate again to get out of the way off the cars...
not trying to confuse you & hope this helps.. its a fair bit of info to absorbe.. but get out on a quiet road.. & try a few things.. practise helps..
& one more bit of advice... braking normally the engine slowing & front & rear brakes all helping to reduce the rate of knots..
BUT.. when you have to stop in a hurry.. the front brake is your best freind.. look at the sports bikes you see doing endos.. (back wheel off the ground) they can do this because the tyres now have great grip & the bikes are lighter.. what this means is.. when you grab that handfull of front brake to stop & the rear starts lifting..100% of the weight is on the front & the rear brake & engine braking is doing nothing at all .. so practice using the front brake.. it will save your life