If this is only occurs when the engine is running, then most likely culprits are air in the clutch hydraulics, or faulty clutch pressure plate. This symptom is known as clutch drag, and is when the clutch fails to fully disengage the gearbox from the engine. If this is the problem you have, you need to take it back to whoever fitted the new clutch and tell them you struggle to get 1st or reverse with the engine running. You may have a faulty clutch pressure plate, in which case, a new one will be required.
First port of call is to renew the hydraulic fluid and bleed thoroughly to make sure there is no air in the system. Sometimes, pumping the clutch a few times before selecting gear helps if there is air or other contaminents in the fluid, and this can help diagnose your problem.
If the symptoms you have are not as above, carry on reading.
If it happens only when stone cold, check gearbox oil level, and/or replace gearbox oil using Honda MTF.
If it happens all the time, possible gear linkage wear and/or adjustment, worn gear selectors in the gearbox, or again, low gearbox oil level.
Hydraulic fluids, gearbox oils or gear linkage problems are relatively cheap fixes, but any internal gearbox faults can get expensive - though this problem with the Jazz gearbox is rare.
