The number 115 is the mass number, so the mass is 115 amu. One amu (atomic mass unit) is about 1.66 x 10-27 kg, so if your looking for the mass of one atom of In-115, multiply 1.66 x 10-27 kg by 115. That's about 1.9 x 10-25 kg.
You chemistry teacher should have taught you this, and probably did, if you were paying attention in class.