Hello,
To secure your router, you need to set it's password. The default password is "
admin". But I would suggest you change the password to something else.
The procedures for all these are long, so I would be directing you to your manual, but in case you don't have the manual, click the below link to view your manual and follow my instructions below.
http://adsl.aruba.it/manuali/3comOCWireless.pdf
To get to the password section, you need to access wizard. Go to page 29. Read and follow instructions to run wizard.
Now, to change the password, go to page 47. Read and follow instructions there.
Lastly, if you forgot your password, go page 105. Read and follow instructions.
I don't think you should have any problems after following the above closely.
Good luck.