Hi Luar Joy Dela Cruz,
Go to End Item(assembly) details page in Windchill 10.1
Go to structure tab, click on report.
There options are available as single level BOM, Multilevel BOM etc.. select one of them let's say Single level BOM.
Then you will get one more window where you have to select the format in which you have to export the BOM for example, - PDF, XLSX, TEXT or XML etc....
is this the expected answer??