To generate diff between two files the command is:

diff -ru original-file modified-file >patch.diff
# eg: diff -ru script.orig.js script.js > bugfix-1234.diff

If you are using IDEs like Eclipse with SVN plugins, then to generate a diff file, right-click on the file and choose Team > Create Patch > Save in File System (browse to the directory of your choice and give the file a name, say patch.diff) and click Next. Choose Selection and click Finish.