Recover disk space from tablespaces

I'm using an 11g database and running out of disk space. How can I determine how much free spaces are in my USERS and UNDO tablespaces? And how can I free these spaces? Thanks for your help.

look at dba_free_space

I ran the following:

select sum(bytes)/1024/1024/1024,tablespace_name from dba_free_space
where tablespace_name IN ('UNDOTBS1', 'USERS') group by tablespace_name;

SUM(BYTES)/1024/1024/1024 TABLESPACE_NAME
------------------------- ------------------------------
0.5606689453125 UNDOTBS1
3.44195556640625 USERS

How do I actually recover the indicated free space?

Thanks in advance.

resize the datafiles

Yes. I found some docs on resizing datafiles.

Thanks for the lead.