There are a number of reasons why your teeth may be discoloured.
The two major types of discolours are:
1) Extrinsic
Extrinsic discolouration is where the enamel (outer layer of the tooth) becomes stained. The most common reasons for this staining is the consumption certain kinds of food and drinks, tobacoo and poor oral hygiene.
2) Intrinsic
Intrinsic discolouration occurs deeper in the tooth below the enamel. The dentin (inner layer beneath the enamel) becomes darker or the tooth gets a yellow tint. The most common reasons for this staining is dental fluorosis, injury, medications, dental materials or dentinogenesis imperfecta.
If discolouration is caused by an underlying dental issues, then this will need to be addressed first before undergoing any form of whitening.