How do you unlock characters on Mario Kart Wii

For most characters, you just need to achieve a gold cup in all the grand prix races. This also usually works for karts and bikes. If you achieve 24 expert ghost staffs in the time trials you unlock the b dasher mk 2 which is a fast kart! If you unlock 32 expert ghost staffs in total, you also get a mii outfit b that looks like Mario's costume.
To get ghost staffs, you have to beat a ghost's record around a course, to do this you have to beat their record by several seconds. Shortcuts on the courses can help cut your time and doing wheelies on bikes can make you go faster, so does performing tricks when you jump.

These are instructions to get several characters in Mario Kart Wii:

KING BOO:
win the 50cc star cup

DIDDY KONG:
win 50cc lightning cup

BABY DAISY:
get at least 1-star rank in the top line 50cc

DRY BONES:
win the 100cc leaf cup

BOWSER JUNIOR:
get at least 1 star rank in the bottom cups 100cc

DAISY:
win the 150cc special cup (crown cup)

DRY BOWSER:
get at least 1 star rank in the top cups.

FUNKY KONG:
unlock 4 fast ghost data on time trials

BABY LUIGI:
unlock 8 fast ghost data on time trials

BIRDO:
play 16 time trials (no win)

TOADETTE:
play all the races on time trials (no win)

ROSALINA:
have a super mario galaxy file then win 50cc mushroom cup
or get at least 1 star rank in the top mirror mode cups

MII OUTFIT A:
win the 100cc special cup (crown cup)

MII OUTFIT B:
unlock all 32 ghost data on time trials