will come in sanskrit

I think it's आगमिष्यामि  

Yes. आगमिष्यामि is a right answer but form will change according to your sentence. आ+गम्-गच्छ is a root word and you can use whichever form you want . For example- आगमिष्यन्ति , आगमिष्यामः, आगमिष्यति। etc. according to your sentence or subject.