If you are proficient in the subject content of the ACT already you may encounter little difficulty. You may try a quick refreasher in ACT prior to see where your skills are prior to testing. Avail yourself of the informative videos and opportunity for ACT specific instruction. Best wishes on your test. : )
The first time I took the ACT, I didn't study at all and still got a 26, which is alright and it gets you plenty of scholarship money to most schools. As long as you're focused and know the basics, you'll be fine. Besides, you can ALWAYS retake it.