Is it illegal to be paid salary when you are not management?

No, it's not illegal at all--many non-managerial employees are paid on a salary basis.

That said, you may still be entitled to overtime when working more than 40 hours in a week. Anyone not exempt from overtime gets overtime; being paid a salary does not itself make a person exempt from overtime. To be exempt, you must be paid a salary AND your job's duties and responsibilities must meet one or more tests for exemption, which you can find at the U.S. Department of Labor (DOL) website. The main tests are the executive (or managerial) test; the administrative test (which applies to certain administrative or office staff who have a fair amount of discretion or responsibility); the learned professional exemption (e.g. engineers, accountants); the creative professional exemption (e.g. some graphic artists); or the computer professional exemption (some IT staff).  If you do not meet one or more of these exemptions (they overlap, so many people fall under several exemptions), you should get overtime even when you are paid overtime.

To calculate overtime when salaried: divide your weekly salary by the number of hours (usually 35 or 40) you are required to work to get that salary; that gives you your effective hourly rate. For all hours beyond 40 which you work in a week, you should receive an additional amount (over and above your salary) equal to 50% of that hourly rate x the number of hours over 40. Say you receive $700 per week and normally work 35 hours: you have an effective rate of $20/hour. Say you work 50 hours one week. You should get an extra 10 (10 hours past 40, since 50 - 40 = 10) x $10 (half your hourly rate), or an extra $100.