Learn Finnish: Minä menen ulos.

Mennä (to go) is a Type 3 verb. To find the stem, you drop the last two letters (-nä) from the dictionary form, which leaves the stem men-. Then, simply add the personal ending for I (-en) to make menen.

Why does mennä have a double n, but menen only has one?

The second n in mennä is actually part of the infinitive ending (-nä), not the verb stem itself. When we drop that ending to conjugate the verb, the second n disappears with it, leaving us with the single-consonant stem men-.

Is it possible to drop the word Minä here?

Yes! Because the verb ending -en in menen clearly tells us that "I" am doing the action, the pronoun Minä is optional. In everyday Finnish, it is very common to just say Menen ulos.

Why do we use ulos for "outside" instead of ulkona?

Finnish uses different words depending on whether there is movement or not. Ulos means "to the outside" and is used because you are actively moving out. If you were already standing outside with no movement involved, you would use the static location word ulkona.