As suggested the fault is relative to a problem within the gear bax- transmission and it can be a case of gears, selectors , counters etc that have worn out but a complete picture can be confirmed only after a first hand view.
Take consensus and estimate if you do not have the equipments and support.

This could be caused by a variety of reasons all which will deal with the transmission. You will have to open it up and check things like your linkage, gears and shafts as well as the chain tensioners. If you don't have the tools or skills It is definitely best to take it to a shop.