Have can be confusing because it’s used in two different ways: as an auxiliary verb, and as a main verb.

• When it’s an auxiliary verb, it’s part of the perfect, and is followed by a participle.  In these cases, it can be contracted:

I haven’t seen James for a while
What have you been doing since we last spoke?
I haven’t got any money on me—can you pay for the taxi?

• When it’s a main verb, it’s either followed by its object or a to-infinitive, and cannot be contracted.

I have to see Mike tonight to loan him a book.
I don’t have anything to do tonight—would you like to get a pizza?
If I had a nickel* for every time someone said that, I’d be rich.
I don’t have any money on me—can you pay for the taxi?

• Also, be aware that have can be both in the same sentence:

Have you had anything for lunch today?
If I had had enough time, I would have been able to do it.

*a nickel is a 5-cent coin

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