Writing data to a file is done with the file functions and there should be some examples in the example finder (Help>>Find Examples) if you search for "file".
Formatting the name can be done with the Format Date/Time String function from the Time & Dialog palette. You can run a loop which will check every X time (let's say once a minute) to see if the date changed (for example, you can check to see if your file exists by wiring its path into the File/Directory Info function and looking at the error terminal) and if it has create your file, or if you want to write throughout the day, you can simply open the file. Basically it depends on exactly what and how you want to write.
To learn more about LabVIEW, I suggest you try searching this site and google for LabVIEW tutorials. Here, here, here, here and here are a few you can start with and here are some tutorial videos. You can also contact your local NI office and join one of their courses.
In addition, I suggest you read the LabVIEW style guide and the LabVIEW user manual (Help>>Search the LabVIEW Bookshelf).
If after going through all that you still have problems, post more details.
___________________
Try to take over the world!
