Crew Accomodations

People need to eat, sleep, bathe, and breathe. If they are going to be on the mission for any length of time, they also need room to move around. The following are my estimates of what the cost in mass and volume are, per person.

Life Support Machinery 755 kg/person Estimated from ISS
Life Support Power 34 kwh/person/day Estimated from ISS
Water Usage 11 kg/day source Estimated from ISS
Food 0.83 kg/person/day source Estimated from ISS
Personal Space 25 m^3/person source How big to make the volume of your habitat

Those values go into the design of the spacecraft, but you also have to account for the mass and volume of trash and sewage people will produce. This mass doesn't leave your spacecraft, but you do have to build an extra volume to hold it all. And that volume requires structure to hold it together.

Trash generated 0.12 kg/person/day source Estimated from ISS, Mainly food packaging
Feces Generated 0.15 l/person/day source NASA Specs
Urine Generated 2.0 l/person/day source NASA Specs
Other Waste Water 9.0 l/person/day source Estimated from ISS. Basically, the water usage per person, minus Urine

For a short duration flight, all of those waste items will probably be collected in a tank. For longer duration flights, while much of the waste can be reprocessed, you will need to have some storage area until the waste reclaimator can do its thing. And a little extra space to handle if the reclaimator breaks down.

For most waste the conversion factor between volume and mass is 1:1. Biological waste is mostly water. Trash is mostly crumpled paper.

Structure

Now that we know the volume of our habitat, we can figure out the mass of the structure that will hold it all together. For the part of the process we can use an old estimation factor from the shipbuilding industry. Steel ships built on Earth generally end up needing about 179.5 kg of steel per 1 m^3 of structure. This includes the framing, the hull, the deck plates and the bulkheads. We can tailor that number up or down to fit our narrative. But it can't be near zero.

As far as what your ship is made out of, odd are it will be steel. Iron is common in the solar system. While aluminum is also plentiful, aluminum is a pain in the ass to work with. Steel is weldable, cheap, and quite strong. There is an formulation of steel for practically every application, and in deep space I'm guessing we would go with a Maraging Steel. It has a high nickel content and it can withstand high and low temperatures without deforming much, or hitting its glass transition temperature.

While this volume -> mass of steel structure estimate seems crude, the alternative is to physically lay out your ship, measure the volume of all of your structural members, and then get the mass by multiplying the volume by the density. This is for an imaginary space craft, so I'm not really that invested in detailed structural work.

Atmosphere

The habitable volume of your ship is filled with air. That air is probably going to be kept at around atmospheric pressure (101 kPa). At that one atmosphere and around 15 degrees C, air has a density of 1.225 kg/m^3. You are also going to need to keep several changes of atmosphere compressed in tanks. While the volume will be smaller, the mass will not. These changes of atmosphere will be needed for normal air processing, recovering from decompression, or clearing the atmosphere after a fire or chemical release. While your life support will have some ability to filter and reprocess fouled air, like with sewage, it may need some buffer to overcome peaks, sags, and malfunctions.