In English, "have" is a verb with two grammatical functions. The first meaning is as a lexical verb, meaning it has a semantic meaning, although a very general one. In this sense, it denotes some form of possession or ownership, either of one object belonging to another, or of something having an attribute. "I have my watch" or "The ice cream has a creamy texture". The second meaning of "have" is as an auxiliary verb, a verb that serves a gramatical purpose. In this sense, it is used in the perfect tenses, to show a completed or permanent action. When "have" is used as an auxiliary verb, it always takes the participle form of the verb after it.
Although native speakers automatically know this, as they know all complicated grammar rules, it can be difficult for a non-native speaker. Some other languages with a perfect tense use a separate, non-lexical verb to form the perfect tense. Others use a form of what would translate as "to be". But because English uses "have" in both senses, we can have sentences like "I have had a horse" and even, ungracefully, "I had had a horse".
Also, there are a few unwritten rules about the differences in uses: for example, "have" as a perfect is contracted to the "I've" (I have) or "He's" (He has) forms. However, as a lexical verb, this is rare: while someone would naturally say "I've ridden a horse", they are less likely to say "I've a horse". There are also a few sentences in which the use of "have" could be either auxiliary or lexical. In "I have prepared papers", it could be either "I possess prepared papers" or "I have completed the task of preparing the papers". Grammar does not always have a perfect description of the way words are used.