Here are some quick ways to stop most problems that may be slowing down your connection:
1. Ensure you're getting a clean test. Stop any ongoing downloads, and shut down any programs that may be using your connection, such as peer-to-peer software.
2. Simplify the picture. If you're using Wi-Fi to connect, try using a wired Ethernet connection to eliminate the possibility of wireless interference.
3. Reboot your modem and router. It's the stock advice, but it solves an astonishing number of problems.
Quora has a great explanation of why restarting your router simply works most of the time.
After running those steps, you should now have enough evidence to contact your ISP if there's a mismatch between your plan and the speeds you're getting. See if they can run diagnostics on their end.
Finally, you can consider changing ISPs if you feel you're not getting what you pay for. You can view the fastest ISPs in your area using Net Index.