If it starts and stalls due to cold weather or any other reason, pull the choke and then start. You will find a knob on left side of carburetor. Pull it and then self start bike. Turn it off after starting bike.
Check the fuel filter. Remove the fuel pipe from filter and see if petrol is pouring out normally.
If problem still persists then carburetor air/fuel mixture requires adjustment.
Regarding gears, clutch needs proper adjustment. Tighten the adjustment nuts properly or clutch will go out of setting again. Since bike is new, over time gears will loosen a bit themselves.