That's only possible if you have national defence obligation, i.e. you completed the compulsory draft or have applied so otherwise. In both cases you need to have citizenship, basic education, of good health, not in jail, no criminal proceedings and a few other minor things. See §81 of the Military Service Act [0].

[0] https://www.riigiteataja.ee/en/eli/513072022006/consolide#pa...