Can't use wireless connection with router

Hello,
Hello,

i just bought a wireless router. but when only one computer is using the internet, it does work. at first , when two computers are using the internet at the same time, only the one using the wireless connection (the one not connected to the router) gets to use the internet while the computer connected to the router can't use the internet

but now, matters have become worse. we both can use the internet but after how many minutes, we both get disconnected from the internet. my sister using the computer connected to the internet still had to reset the internet everytime and it takes too long before the internet works again. help, please?