gear changing

Hello

I've just bought my first ever motorbike (a t80) and as a complete novice I need some advice. My t80 has no clutch lever and the man I bought it from said that it is an automatic. I change gear with my left foot as I ride along but sometimes it sounds as though the bike is changing gear by itself. So should I be changing gear or should I let the bike do it? I know that this is probably a silly question but I don't know what is the best thing to do and I want to ride it properly.

The T80 is semi automatic with centrifugal clutch, and you change the gears with left foot using toe & heel pedal. Toe for going up the box Heel for going down. There are 4 gears with neutral below first (keep pressing with heel till no more gears & you are in neutral.
Try to get a user manual, or buy a Haynes manual which will full explain