You need to pick it wisely as Microsoft will try to get as much money from you as possible because it costs 10 dollars to change it.
Also there are unwritten rules when it comes to creating your gamertag.
Rule #1: It may contain at least 1 marijuana reference.
Rule #2: It may or may not (your choice) reference alcohol or boobs.
Rule #3: It must have 3 x's somewhere in the gamertag (perferably at the beginning and the end)
Rule #4: If you are 12 years old and are planning on playing halo 3 all day after school until your bedtime you must put MLG in the beginning of your gamertag.
Rule #5: You must, MUST make it look you're so retarded that you spent your whole time as a baby in a washing machine.
xXx LeGiiT SniiPeZ
PRO SN1P3R BR
Sw4gga like dis
MLG 4 SH0T
iTZ iLoGiCzzz v2
The list is endless
II TaLeNTz v3
x NeRv3 x
iTz 2 Pr0
o iPRoShoT xX
"I got a new gamertag today."
"My gamertag is so pro x MLG (joke)"
"Whoa, that gamertag is leet"