For many years I've been creating many databases in it. Only a day ago one of it stopped opening, I checked up other database, it were valid. I decided to fix dbase by the facilities, some programs from Google and I finally I got - First.dbf is not a table. I still can't understand what I done.

I assume you have no backup and you are running in NOARCHIVELOG mode, otherwise you wouldn't be asking this question.
Assuming that your db file is really corrupted, then you can startup the database without this file:
Also you may apply following articles, guides and etc…
https://support.microsoft.com/en-us/help/193952/how-to-troubleshoot-to-resolve-suspected-corruption-in-visual-foxpro
https://download.cnet.com/Recuva/3000-2242_4-10753287.html
https://www.fixtoolbox.com/dbffix.html
1. Take a full backup of all your current db files, control files, redo log files.
2. Startup mount the instance
3. ALTER DATABASE DATAFILE 'fullpath/filename' OFFLINE DROP;
4. ALTER DATABASE OPEN;
This will drop the file from your db, so all data in it will be lost, but the data in other tablespaces will be accessible.
If this doesn't work, restore to its original state using the backup taken in step 1.
However, before concluding that your file is corrupt, try to see what happened to it.
Post the following info:
Your archivelog/noarchivelog mode.
Tablespace to which this file belongs.
Oracle error when starting up the database.
Also, startup mount your db and :
SELECT * FROM V$DATABASE;
SELECT * FROM V$DBFILE;
SELECT * FROM V$RECOVER_FILE;