First, you'll have to download a Vietnamese input method. I recommend Unikey. If you can't understand the program's options then click on the down arrow and choose English.
Then you'll have to learn how to type Vietnamese. The two most popular methods are TELEX (popular in the North) and VNI (popular in the South).
Here are the rules of TELEX: aw=ă, aa=â, dd=đ, ee=ê, oo=ô, ow or [=ơ, uw or w or ]=ư, f=huyền, s=sắc, r=hỏi, x=ngã, j=nặng. Examples: Thor=Thỏ (heh), sex=sẽ, thuwowngr=thưởng (you can type the accent mark after the word; Unikey will automatically put it in the right place), xooong=xoong (the third o cancels the ô, the same goes for other letters and accent marks).
Here are the rules of VNI: 1=huyền, 2=sắc, 3=hỏi, 4=ngã, 5=nặng, 6=^ 7=? (Ơ, ư), 8=ă, 9=đ. Examples: la1=là, co6ng3=cổng.
Hope this helps!