If you want an ACSM certification you have to get the manuals and then take a one or 3 day course for certified personal trainer. There is a hefty exam at the end and you have be really know your stuff. do you know what actin and myosin are? The krebs cycle? ADP v.s. ATP? etc.etc. It is a major test and no joke. You have to be familiar with understanding VO2 max, senior fitness concerns, pregnancy, cholesterol levels. Cardiac rehab stuff....on and on. It is totally passable but do not take it lightly.
There are easier certifications to attain such as the ACE exam. You can get the manual and then sign up to take the test.