What could be causing my knee pain?

I injured my knee when i was 14 playing soccer, i remember hearing it pop. It bothers me a lot now 6 years later. I never really got it checked out because i thought that the pain would go away, that i just needed time to let it heal up a little bit. The pain eventually went away but then i started noticing that my knee was giving out when i walked especially when i went up stairs. And these past 2 years it has gotten pretty bad, it gives out all the time, it pops like crazy when ever i move it, and i get these sharp pains in the left side of my knee that feel like they are down IN my knee. I havent been able to get it checked out because my insurance couldnt cover a specialist. Now i have a lot better insurance and can get it checked out but i dont want to go ahead with this unless there could be something seriously wrong. I did the symptom checker on webmd and it said that it could be my ACL joint and thats what my finace said and I trust his opinion since hes had a knee replacement and his other knee is bad. And I know that ACL tearing is common in sports injuires.