The character customiser can be accessed after you have completed the 2nd mission it is located in the avengers mansion down the right side of the stair case and in a room in the back left of the command room, Or you can press the start button and select "Go To Customiser". Note that the character customiser works different in this game then previous LEGO titles but is fairly simplistic.
Update -- WARNING: I used all these codes and as a result my game has glitched, aswell as a few other people I know have hadthis problem. What the glitch is, i have "unlocked" all characters but do not have the "Time waits for no Kang" achievement AKA All races. A friend of mine has said this happened to him and he got it to pop by re-racing all the races in which one of the characters unlocked by a code is the prize for winning so if you want my advice do every race twice.
Also as an added bonus try these codes for extra characters. Codes must be input in the computer in Gwenpool's room (top floor on the left) inside the avengers mansion.
BCR7QJ - Ant-Man
QG3VH9 - Baby Groot (Ravager)
M68P3L - Captain Britain
CDS278 - Crimson Dynamo
S947TP - Darkstar
GAVK9R - Giant-Man
LBYT59 - Grandmaster (Thor: Ragnarok)
XG7SAL - Green Goblin
G6K2VM - Hawkeye
NCMJU4 - Hellcow
5G7HFS - Hulkling
JDNQMV - Loki & Classic
HCE926 - Maestro
UUTZNC - Militant
BK9B3Y - Misty Knight
HL7L7Y - Ragnarok
JD9GQA - Scarlet Spider
D6LJ4P - Songbird
RMADXF - Spider-UK
CW9BRS - Spider-Woman
4U9DAT - Vision
7KDY3L - Vulture
8KD3F6 - Winter Soldier