a portfolio for the Interior Design department of a college shouldn't be so hard .Cos at the end you have zero experience in designing right ? so you have to create a visual or digital portfolio of whatever you designed till now..it doesn't need to be professional but at least you have to show that you have the TALENT !! it can be your basic sketches, black white freehand drawings ,simple drafting samples if you ever experienced ,maybe something you designed ( a wall ,painting ..etc ) ...etc..so any material that connects you to be an interior designer can be appreciated in this portfolio..But don't forget whether it's a digital portfolio or visual pront-out ,you have to design your portfolio as well !!! that's soooo important more than what you have inside..if it's digital use sofwares like photoshop ,create nice templates and should be continious from one page to another ; if it's a print out they have to be same size nicely covered ,choose nice colors !!
if you don't have any material for instance ,please start to create your own portfolio pages...start a drafting course ,or an art course..paint ,color ,design anything,take lots of pictures around..etc..collect your sources then it'll be easier when it comes to applying ..
good luck !!