For the ACT writing section, logic is very important. It is essential to connect your thoughts together coherently, whether it be with skillfully used transitions or simply by association. Examples are also crucial. You can use examples from your daily experiences, current events, books, whatever. Just make sure that it backs up your thesis (which should be the last sentence of your first paragraph). Keep in mind, however, that personal experiences and logic will be your main tools of argument.
To transition to a counterargument, you can use phrases like "Other people, however..." or "There are many out there that believe otherwise..." or even the use of the word "however."
The 3 paragraph body concept works well, though if you can add more examples, that would work as well.
Rumors are are that if you can fill up 3 or 4 pages with coherent and logical writing, you are almost guaranteed a double-digit score.