The easiest way to come here is by airplane. Kota Kinabalu has an international airport with two terminals, one for regular flights and another for budget airlines.It is connected from most major cities around South East Asia and North East Asia. Brunei, Kuala Lumpur, Manila, Singapore, Hong Kong, Osaka, and Seoul are some major cities that have connecting flight to and from Kota Kinabalu.
Being on the Borneo Island, Kota Kinabalu is disconnected from the mainland Asia, and can be accessed by land only from within Borneo Island.There is a good road link from Brunei via Limbang, Lawas (Sarawak), Sindumin (Sabah), Sipitang town, Beufort, Papar and toward Kota Kinabalu. This Journey will take about 6 to 8 hours.
KK is a port city and does receive Ocean liners. I am not sure if any known Ocean liners make a stop here. There are ferry services from Labuan to KK, and vise versa, daily. This journey will take 3 hours.From Brunei you need to take a ferry to Labuan first (about 1.5 hours)before proceeding to KK by another ferry.

There are also ferry services from Nunukan, Kalimantan, Indonesia to Tawau.