How can I fix my cars problem ?

I have a problem with my cars gearbox,,which is ''Peaugeot 307 2006'' ?? when I start the engin & I want to start going afew miles later the warning signal is apeear from the scren,, befor 3 monthes I cheked my car, it was said its a problem with gearbox,, I Don't know how can I fix this problem,,cause its really unpleasand I can't going to a far way the gear can't stand it,,, I've to turn off the engin many times till the gear will be hot and start going without problemm,,,or somtimes the warning signal is apear when I am in driving above 100 km/h,,,please help me what should I do about it ??! thanks & have a nice time :)