What causes my signal problem?

I use a modem to surf around the net and the speed of the modem is 3.6 Mbytes. The problem is that it takes longer time to work or when I want to upload something. I just have to reload the page again and again,and I just don’t know what is the problem. Is it because of the weather?(cloudy/rainy),or problem from the server itself? Will I make it slower if I open multiple sites at once?