The basis of if statements is that you test to see IF something is equal/bigger/smaller/other condition than something else. If it is you do something. And maybe you want to add an "Or if it isn't do something different" part which is the "else" part of if/else.
So it looks something like:
if (sensorValue1 > 200) // or sensorValue1 <= sensorValue2 or all sorts of other comparisons
{
Do something like maybe switch something on
}
else
{
do something else like maybe switch the thing off
}
Which part of that is causing you trouble?
Steve