The present perfect tense is mostly used in two situations: to indicate that an action happened at an unspecified time in the past, or to talk about past events that still have a connection with the present.
The present perfect is formed by combining the present tense form of the verb "to have" and the past participle of the verb. It is often used in conjunction with time expressions denoting an unspecified time in the past, such as "ever," "never," "many times," "already," "yet," etc. Since the present perfect is only used when the time is unspecified, it cannot be used with time expressions denoting a precise moment in the past (such as "yesterday," "last week," "when I was a child," etc.).
When used to talk about an unspecified time in the past, it's often helpful to think about it in terms of "experience." For example, saying "I have been to Japan" can be understood as "I have the experience of going to Japan." The exact time is not important, as the emphasis is on the action (i.e. on the experience). When used to underline the connection with the present, the past perfect indicates that an action started in the past and has continued until now. For example, "I have loved pizza since I was a little kid" indicates that the action has been going on for a long time, and it is still highly relevant to the present.