If its a 16v you can check the transmission by selecting at the so-called 'tower' mechanism on the gearbox. Look for the balance weight on it, which looks like the head of a golf club. It has the two cables from the gearlever inside the cabin attached. The lateral cable adjustment might have slipped. When this happens, then basically the gear lever and gearbox selector are out of phase, and selection problems occur.
Get a gear linkage repair kit, quite cheap and change gearbox oil. In meantime select 2nd gear before going into 1st from start off. Other then that could be a few things like worn bushes etc. Most likely your gear linkages need aligning with the tool or clutch might need adjusting/tightening. Do the inexpensive things first or get diagnosis from a good garage.
