Input power & Torque to a DC Motor
You would have an easier time if you knew the motor constants as I describe in detail to my answer to this question: How to improve torque and RPM of a DC motor? You should be able to get those from the motor manufacturer and they would take into account the losses mentioned by the others.

But you can get a ballpark estimate given what you have. Given the voltage, armature resistance and Ohm's law, you are able to find the current and therefore get the input power.

You can then determine torque at a known speed with the following equation:

\$P_{in} = P_{out}\$
\$V \cdot I = \tau \cdot \omega\$

Where:

\$\omega = \text{angular velocity in radians per second}\$

So you would just need to convert RPM to rad/sec using:

\$ \dfrac{RPM \cdot 2 \cdot \pi}{60} = rad/sec\$

Then divide your input power by your angular velocity to obtain your torque at that speed.