You can upload from an HTML form.
There is a standard HTML form field that is for file uploads, i.e. it lets the user browse their local machine for the file they wish to upload. The form must use the POST method to upload the file, if you use the GET method you will only send the file name NOT the data.
You must also specify the encoding type for the form.
In the action for the form give the name of your servlet.
Within your servlet you will need to know how to parse posted data to strip out headers and new line characters added by the POST process. Once you have done this you will have your data on your server from where you can manipulate it, save it for file / database.
It is not the easiest thing I have learned, I would suggest you search the Sun Java site. there are tutorials on how to parse POST data which will help you.