To tell that you've done or experienced something at some point in your life, without being interested in communicating the time it happened, you can use the construction plain past verb + こと が ある.
| I have written a letter in Japanese before. | 日本語 で 手紙 を 書いたこと が ある。nihongo de tegami o kaitakoto ga aru. |
| I have swum in a river before. | 川 で 泳いだこと が ある。kawa de oyoidakoto ga aru. |
In this grammar, こと nominalizes the verb, turning the verb X into something that can be translated to the event of X, and by adding が ある, we get a phrase meaning something like there exists the event of X.
| I have climbed a mountain before. | 山 を 登ったこと が ある。yama o nobottakoto ga aru. |
| I have seen fireworks before. | 花火 を 見たこと が ある。hanabi o mitakoto ga aru. |
| I haven't (ever) traveled by an airplane. | 飛行機 で 旅行したこと が ない。hikouki de ryokoushitakoto ga nai. |
| Have you never drunk (cow's) milk? | 牛乳 を 飲んだこと が ありません か。gyuunyuu o nondakoto ga arimasen ka. |
