As a DO you will need to take COMLEX 1 usually after you complete 2nd year. For NYCOM it is a requirement before starting clerkships.You could then take Step 1 at around the same time which is ideal due to the amount of time you will have already invested in COMLEX 1.
Step 2 is usually done after 3rd year. You may complete it before the September of your 4th year if you want the scores available for the interview invite part of the residency applications.
As a DO if you do well on step 1 there may be no need to take Step 2 but you need to look at the field and residencies of your choice for more guidance on that.