i have a decent wifi connection but for some reason my downloads are really slow this didnt happen before and i dont know what

Other reasons that your speed has slowed could be:

If you connect multiple devices to one wifi they share bandwidth, have you added any new devices to your network?

I would assume that your wifi is protected (has a password) if it is not then you could have some neighbors stealing wifi, and therefor bandwidth, from you.
Do you have a "guest mode" set up on your router? Guest mode is something that allows visitors to connect to your wifi without having to give them your normal wifi password. However some routers do it in a very unsecured way and if you have left it at default settings then other people "neighbors/strangers" could potentially connect to your wfi without you knowing. I would recommend turning that feature off!

Just how strong is your wifi signal? Is it lower than it use to be? Low signal strength can lower speed.
Have you noticed any new wifi clients in your area? You might have to go into your router settings and change the channel to avoid interference.
Have you moved your router recently? Walls and appliances can interfere with reception.

Do you reboot your router regularly? Sometimes they just need a refresh just like you would reset a phone or reboot a pc. Some routers have a reboot button but most you can just unplug them for several seconds. Rebooting the devices that are connected to them after a reboot is also a good idea.

You can also try flushing the dns from your pc. To do that press start, type in cmd, right click on it when it comes up and run it as an administrator,
Type ipconfig /release in the command prompt.
Wait a few seconds for a reply that the IP address has been released.
Type ipconfig /renew in the command prompt.
Wait a few seconds for a reply that the IP address has been re-established.
Type ipconfig /flushdns in the command prompt.
Type exit.

If all else fails you can call your ISP and have them do a refresh from their end.

As the resident IT person in my family (>_<) for the last 15 years I have found routers/modems to be the most troubleshoot heavy pieces of tech bar-non!!

[Side note]Make sure that you have actually set up your router software with an admin/password for accessing its firmware. The default is almost always: user[admin] password[password] which means if you happened to have left your network open then anyone could connect to your router software and change settings.