It can't be "cette", because it is a demonstrative adjective qualifying a noun, not to be used as a stand-alone:

• cette robe est bleue

http://french.about.com/od/pronunciation/a/r_2.htm this helped me

Demonstrative means that you are showing something : "this is my watch", for example = c'est ma montre. A demonstrative adjective is placed in front of a noun : this watch = cette montre

In English, you have "this" and "that", but in French, you have several forms for the demonstrative adjective, depending on the gender and the number of the object you are showing:

• ce portable (this/that cell phone) - masculine
• cet ordinateur (this/that computer) - 'T' is added to "ce" to liaise with a word starting with a vowel
• cet homme (this/that man) - same addition because of the non-aspired H
• cette table (this/that table) - feminine
• ces portables, ces ordinateurs, ces hommes, ces tables: plural form for both genders

This T liaison is required, though.

yes "c'est" is always used for he/she/it + is + nominal group (article + noun + adjective optional)

the form changes to "ce sont" for they + are + nominal group