Simply follow these steps:

1. Log into your GMail account
2. Now goto your settings panel by clicking on the ‘Settings’ at the top right hand corner of the page
3. Now you are in settings panel, select the ‘Filters’ tab
4. You can see any filters you have created there, If not yet, lets create one to filter out our unwanted mails. Click on the ‘Create a new filter’ link at the bottom
5. Whooshh.. Create a filter panel opens above. Now we have some options. If we know the email id of this guy who sends us the unwanted msg, you could simple enter that id in the field ‘From’ or if you don’t, don’t worry we have more options.
6. We could even block a mail that was also send to some other email id. For example consider you have 2 mail ids you gave both to everyone, what if the other guy sends his mails to both of the accounts? It duplicates right? So you could say, enter anyone of your mail id at the ‘To’ field. Thats it!
7. Another option, Consider you want to block mails with common subject like : ‘Facebook Invitation’, ‘New Offers’, ‘Orkut Invitation’, etc.. just enter the words that comes in as the subject to the ‘Subject’ field
8. Or you could also make that case to the content of the mail. Just enter words to catch in the ‘Has the words’ field. Or vice-versa in the ‘Doesn’t Have’ field
9. Remember that you set the right criteria before proceeding, you always check whether you were right by clicking in the ‘Test Search’ button
10. Click ‘Next Step >>’ when you are done with filling the filters
11. Now.. you are taken to ‘Part B’ of Filtering mails, here you have to select – what to do with the filtered mails. You have some range of options, you can skip it from inbox, mark it as read, apply star, or add labels, or forward it to another mail id, and you could delete it automatically whenever a mail with filtered options received. And finally you can set ‘Never send it to spam’ its only required if your important mail is constantly send to SPAM folder automatically.
12. So when you are done with your choice just click ‘Create Filter’ button. Note: If you also want to delete the current mails which follows the selected criteria check ‘Also apply filter to ## conversations below’ before proceeding.

Thats it!!

Congrats.. you have successfully created a filter that filters your inbox. Enjoy!