Train reservations to and from Nice

For TER trains in that part of France you do not need reservations. Just a ticket, which you can buy at the station.

To go to L'isle Sur La Sorgue however the fastest trips use a TGV for part of the route, and for those you need a reservation. You can again just get that at the station, but booked in advance is usually cheaper.

You can go by TER train from Nice to L'Isle sur la Sorgue. You would take the TER to Marseilles and then change there for the train to L'Isle sur la Sorgue. If you do that, no reservation is required. That trip will take about an hour longer than the trip on a TGV but it could be as little as 35 minutes less than the time of the fastest TGV. Note: at some point, you will have to have a stop over and get on a TER to get to L'Isle sur la Sorgue and the length of time for that layover varies. There is a TGV that leaves Nice at around 7 AM and you would change in Avignon for a TER and arrive in L'Isle sur la Sorgue at around 11:30. About an hour later there is a TER that leaves Nice at around 8:30 with the switch occurring in Marseilles and you arrive at around 1:40. It really depends on the time of day that you want to travel. I am assuming that you are not traveling with a Eurail pass and are not asking about TGV reservations for pass holders.