I am using the 3750G catalyst switch and I want to configure netflow but when I put the first command it appears that netflow is unknown: invalid input detected when i'm using netflow
Not all Cisco devices support all features. In addition to there are also differences between IOS versions feature sets. You have to check the documentation for the exact hard- and software version you are running.
A quick search on google showed that the 3750G does not support Netflow.
One possible workaround might be to setup a monitoring port and setup a Linux box to capture the traffic an export the Netflow data. This will only work as long as you don't exceed the the maximum capacity of the switch port your capture box is connected to.