I have just installed Slackware. I'm logging in as root. I want to
create new user account. How to create new account?

Use the command "adduser".

I want to add that kind of user who doesn't have rights that root has.But
He/she can access CD, DVD, USB, except root files and other users directory. What should I do?

You'll want to use the "adduser" command like Nylex said. This will create a more limited account. You will be asked what "groups" the user should be in such as "cdrom", "audio" etc...It's covered at the end of the "installation HOWTO"..you can find it in one of the stickies in the slackware forums

Solitude:

if you are running slackware 12.2 you should have available gui based user management applet. just click lower left ribbon menu button>System>KUser this should offer you the gui. there click User> Add> write your user name. Highlight the newly added username click Edit. There you can add or subtract group memberships to the new user.

add for him groups like: cdrom, audio, and games (the user will have enough to enjoy); set group "user" as his default group or Set as Primary group.

if you want him to have access to usb-pendrive, disk, or mount drives add him groups "messagebus" "daemon" "haldaemon" "plugdev" (don't do this if you don't want to).

You may also use terminal command "adduser" as Nylex suggested. for more info on how you manipulate the command read the manual: "man adduser" <Enter>

hope this helps.

goodluck.

When you use adduser, it'll ask you if you want the user to be a member of groups in addition to the default "users" and will present you with a list of recommended ones. Those should be sufficient, IIRC.

Easy way,
example
useradd -g users -G slocate,disk,cdrom,plugdev,lp -d /home/okos/ -s /bin/bash -m okos

Modify a user. Add additional groups for an existing user.
example
usermod -G ftp okos