Learn Finnish: Minä nousen ylös.

The dictionary form of the verb is nousta, which means to rise or get up. To say "I get up", you take the verb stem nouse- and add the personal ending -n for "I".

What type of verb is nousta and why does the "t" disappear?

It is a verb type 3. With type 3 verbs ending in -sta or -stä, you remove that ending and add an -e- to make the stem. So nousta becomes the stem nouse- before you add the personal ending.

Do I always have to say minä?

No, you can simply say nousen ylös. Because the verb ending -n already clearly means "I", the pronoun minä is optional and often dropped in everyday spoken Finnish.

If nousen already means "I rise", why add ylös?

While nousen means "I rise" or "I ascend", adding the direction word ylös (up) creates the specific phrase "to get up" (such as getting out of bed in the morning). It works very much like adding "up" to "get" in English.