I'm a tight budget person, please suggest me the cheapest mode of transport to visit hongkong.

First of all, you have enough time to visit major attractions in HK.
Disneyland Resort, Ocean Park, Victoria Bay, Tsim Sha Tsui & A Symphony of Lights, Victoria Peak & Madame Tussauds, Ngong Ping 360 & Big Buddha, Central Area, Causeway Bay, Ladies Street and Lan Kwai Fong are must sees. You can go to Kowloon City, SoHo Hollywood and Hung Hom to taste local snacks and make friends in bars.

In addition, if you wanna stay in there for 13 days, Ordinary Octopus Card is recommended. You will enjoy some discounts when taking public transportation, such as bus and subway. You can also buy some goods at convenient store by using it.
For adult, the deposit is 50 HKD and initial stored value is 100 HKD.