In Windows DOS, you need to run db2cmd before you can do any db2 command. If you want to run multiple commands, usually you can output the commands into a file and call db2cmd like this:
db2cmd -c db2 -tvf cmd_file.sql
But if you don't want to generate a file, here is how
db2cmd -c db2 connect to mydb user me using passwd && db2 select * from my_table && db2 terminate