How to buy a gift for my father??

Do you mean where to buy a gift or what to buy? You need to supplement your question and be more specific.

maybe he has a hobbie you could get him something connected to that?
or maybe he is a DIY man inwhich case tools never go amiss.
whats he into, maybe you could have a look on Ebay for some movie memrobilia or something connected to his fav movie.

Anything bought and it looks like you have really considered what they like or need. It doesn't have to be expensive as long as it is bought from the heart and not out of convenience.

When in doubt buy something that they use regularly and costs money. It could be a favourite deoderant,razors,even socks or paperback by their favourite author.
This type of gift is never shoved away and forgotten. After all sooner or later they would have to go out and buy it for themselves.