If it rains tomorrow, I won't have to water the garden. The speaker suggests that there is a real possibility of rain.
If it rained tomorrow, I wouldn't have to water the garden The speaker sees the possibility of rain as being more remote.
In present situations, we also have two possibilities.
If it is raining now, I won't have to water the garden this afternoon.. The speaker does not know whether it is raining or not. (S/he is presumably indoors).
If it were raining now, I wouldn't be watering the garden. It is raining, and the speaker is watering the garden.
There are several articles on conditional sentences here.
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