First, generate the key pair on the host that you are connecting from.
ssh-keygen -t dsa
Accept defaults.. can put passphrase if you wish for extra security.
Copy id_dsa.pub to the remote host somewhere. Then add it to the authorized_keys file under the home directory of the user that you are trying to log in as.
For instance if you are wanting to send a file to the remote server as user 'bob'...
copy the id_dsa.pub file to /tmp on the remote server
log in to the remote server as bob
cat /tmp/id_dsa.pub >> ~/.ssh/authorized_keys
Now you should be able to ssh or scp something to the remote host and not be prompted for a password.