The First Conditional
The first conditional has the present simple after 'if', then the future simple in the other clause:
if + present simple, ... will + infinitive
It's used to talk about things which might happen in the future. Of course, we can't know what will happen in the future, but this describes possible things, which could easily come true.
If it rains, I won't go to the park.
Second, we can use it to talk about something in the present which is impossible, because it's not true. Is that clear? Have a look at the examples:
If I had his number, I would call him. (I don't have his number now, so it's impossible for me to call him).
How is this different from the first conditional?
This kind of conditional sentence is different from the first conditional because this is a lot more unlikely.
For example (second conditional): If I had enough money I would buy a house with twenty bedrooms and a swimming pool (I'm probably not going to have this much money, it's just a dream, not very real)
But (first conditional): If I have enough money, I'll buy some new shoes (It's much more likely that I'll have enough money to buy some shoes).