How do you name #"CH"_3"Br"#?

Well, you need to know what the name of the alkyl chains are, and the stem for the substituent.

• Alkyl groups are chains of #"CH"# bonds, such as #"H"_3"C"-("CH"_2)_n-#.

The most common ones are:
#"Meth"-#: one carbon in the main chain
#"Eth"-#: two carbons in the main chain
#"Prop"-#: three carbons in the main chain
#"But"-#: four carbons in the main chain
#"Pent"-#: five carbons in the main chain
#"Hex"-#: six carbons in the main chain
#"Hept"-#: seven carbons in the main chain
#"Oct"-#: eight carbons in the main chain
#"Non"-#: nine carbons in the main chain
#"Dec"-#: ten carbons in the main chain

• Substituents are non-hydrogens, such as #-"Br"#, #-"Cl"#, #-"OH"#, #-"NH"_2#, etc. They each have their own stems, such as bromo, chloro, hydroxyl, or amino, in that order.

You have at least two ways you can name #"CH"_3"Br"#.

• Bromo + meth + ane, because a bromine substituent (bromo) is on a one-carbon alkyl chain (meth-), and the alkyl chain has no double or triple bonds (making it a haloalkane compound, specifically a bromoalkane compound, as compared to a bromoalkene or a bromoalkyne).
• Meth + yl brom + ide, because the one-carbon chain has the prefix "meth-", the "yl" is for hydrocarbon chains that are not terminated on the second end by a #"CH"_3#, and bromide is the name of the #"Br"^(-)# anion.

So, two possible names are bromomethane or methyl bromide.