As I understand it, you have the router in your room, which is where your computer is sitting, and you're still getting connection failure from your wireless signal. Is that right? And, is your sister's computer in another room, and is she getting a good signal? That's odd; you should have a strong signal if your computer is in the same room as the router.
If the router is in your room with your computer, connecting by wire is fairly simple. You have some ports on back of your router for cable connections, right? And, on the back of your computer, do you have a connector of the same type, called an RJ45 jack? Just get a CAT5 cable and connect it to the router and to your computer. If your computer does not already have an RJ45 jack, you'll need to install a network card, such as
D-Link Desktop 10/100 PCI Network Card, in a PCI slot in your computer.