Why is my internet so slow?

I pay for 20 megabite internet but everytime i run a speed test on my desktop it shows like 2 megabites. It's not as bad on my phone tho because i get like 13 on there but still not what im paying for. No one else was using the internet btw and i dont have a modem so i can't use ethernet with my pc. HEEEEEEEEEEEEEEELP