depends on how much data you want. go to different services and just talk about prices. talk about your needs and what you use your phone for. look at sprint, at&t, verizon, or others. shop all their deals and choose what's best for your budget. it may be cheaper to keep on your parents plan and just pay them your share.
if you're on WiFi a lot or almost all the time, you may not need unlimited data. just get one with a couple GB, and you should be fine. some plans have it where you can roll over unused data each month for the year.
if you know you're not going to be on wifi, download entertainment while on wifi to watch offline so you don't use data. streaming music uses barely data at all.
there are a lot of variations for a phone plan. so it's hard to say exactly what it will cost. plus you have to think if you want a contract, month to month, or some other plan.