my computer is running extremely slow, especially the internet. I don't know why, but I need it fixed. Help?

Go step by step and test.

1. Power cycle

Power off the router. Unplug it from the wall. Wait a while.

Plug it back to the wall. Power the router on. Wait until all the lights are lit properly.

It will take a while.

Restart the computer.

Start up in Safe Mode

http://support.apple.com/kb/PH11212

2 Empty Caches

Safari > Preference > Advanced

Checkmark the box for "Show Develop menu in menu bar".

Develop menu will appear in the Safari menu bar.

Click Develop and select "Empty Caches" from the dropdown.

3. Safari > Preferences > Privacy > Cookies and all other website data:

Press “Remove All Website Data” button

4. Turn off Extensions if any, and launch Safari.

Safari > Preferences > Extensions

Please answer as many of the following questions as you can. You may already have answered some of them; in that case, there's no need to repeat the answers.

• Restart the router and the broadband device, if they're separate. Any change?
• Run Software Update and install all available updates for OS X or the computer's firmware. Any change?
• If possible, connect to the router with an Ethernet cable and turn off Wi-Fi. Any difference?
• Are any other devices on the network, and if so, are they the same?
• If you can connect to more than one network, are they all the same?
• Are all network applications affected, or only some? If only some, which ones?
• Is networking always the same, or is the problem intermittent?
• Disconnect all other devices from the network. Any change?
• If you're running a MacBook with the lid closed, open the lid. Any difference?
• Start up in safe mode and test. Any difference?
• Start up in Recovery mode. From the OS X Utilities screen, select Get Help Online. A clean copy of Safari will launch. No plugins, such as Flash, will be available. Any difference?
• If possible, turn off Bluetooth and disconnect any USB 3 devices. Any difference?