Learn Hardcore Finnish: Minä soitan kitaraa kotona.

Yes. It’s completely natural to say Soitan kitaraa kotona. The -n ending on soitan already marks the subject as I. Use Minä mainly for emphasis or contrast. In spoken Finnish you’ll also hear Mä soitan kitaraa kotona.

What verb is soitan, and how do I conjugate it?

It’s the 1st person singular present of soittaa (to play an instrument; to call on the phone). Don’t confuse it with soida (to ring/sound), which is intransitive.

minä: soitan
sinä: soitat
hän/se: soittaa
me: soitamme
te: soitatte
he: soittavat

Why is it kitaraa and not kitara or kitaran?

Because soittaa + instrument takes the partitive case by default: soittaa kitaraa, soittaa pianoa, soittaa viulua. The partitive signals an ongoing, open-ended activity rather than a completed, countable result. Using kitaran would suggest a bounded, total object, which doesn’t fit the meaning of simply playing an instrument. The nominative kitara is not used as a direct object here. For inherently plural instruments you’ll see the plural partitive: soittaa rumpuja (play the drums).