>> 1) Draw all the possible structures for chloromethane and name each?
You're right, there is only one molecule with formula CH3Cl, which is chloromethane. I believe that question is supposed to be, "Draw all the possible structures of the chloromethanes" (plural) meaning chloromethane, dichloromethane, trichloromethane, and tetrachloromethane. The common names are methyl chloride, methylene chloride, chloroform, and carbon tetrachloride.
>> 2) Draw all the possible isomers for:
>> a) 1,2 -dibromocyclohexane
>> b) C3H4Br2 (cyclic only)
In both cases there are three (one cis and two trans). The trans isomers are mirror images of each other, much easier to see using models. They are like your left shoe and right shoe. They look the same at first, but are really different. The mirror image of the cis isomer turns out to be the same molecule, so there is only one cis isomer.
Steve