Use media queries to target the css to specific devices.
In your case assuming that you want to make the menus horizontal when you resize the window and the window size is less that 980px, declare the styles to be applied for all the devices with width below 980px within the below declaration
@media screen and (max-width: 980px) {
// write the css here
}
To make the menu horizontal try this
@media screen and (max-width: 980px) {
#lateral{
float:none;
width:100%;
}
#desc_fantas p{
height:auto; // you have declared 100px height for this initially, i just changed it to reduce the height when the menu is horizontal. Change it as you need.
}
#menu-o-menu{
text-align:center;
}
#menu-o-menu li{
display:inline-block;
}
}