1) How can I delete a file via keyboard shortcut?
2) How can I lock my screen via keyboard shortcut?
From here, to send a file to trash, the key combo is
Locking screen is a little more difficult since there isn't a built in way to do it. However if you use Expose along with keystroke (i.e. F12 activates screen saver) and also set Security settings to ask for password anytime it returns from the screen saver, you will be able to accomplish a keystroke binding to lock your screen.
You can access keyboard shortcuts for Expose in System Preferences -> Keyboard Shortcuts -> Expose & Spaces. You will also want to check System -> Preferences -> Security to enable the setting that always asks for a password on resuming from screen saver.
Locate the Screen Saver preference panel
Click on "hot corners"
Set a hot corner to start the screen saver.
Assuming you set the screen saver to require a password (in Security) you can move your mouse to the hot corner to start the screen saver, in essence locking the computer.
If you fear hitting the locking button unintended, there is a Dashboard widget for this: http://www.apple.com/downloads/dashboard/networking_security/inticlock.html
1) In Finder, this is Command-Delete.
2) Enable the "Require password immediately after sleep or screen saver begins" option under Security in System Preferences.
You can now use this combination to lock the screen: Ctrl-Shift-Eject.