Sounds like you're going to have fun. The prop department can be loads of fun - but it's always loads of work, too. You will often be the person who is closes to the actors, the sets, the camera, working very intimately with the details of the production and making sure it all stays real and easy.
Your subject header says "Propmaster" but some of the advice seems to be aimed more towards a prop fabricator. I'll try to address at least a tiny fraction of the "Propmaster" part of it...
Depending on the budget, a good contact at a prop rental house is essential. If you have to build a lot of things, a good contact at a trusted shop that has s CNC router, as well as the usual building tools is essential... a good graphics person with access to a sign shop that can print on more than just paper, and has a vinyl cutter and perhaps a CNC router that can cut PVC, etc... The ability to be resourceful and confident on set. If you have to build things yourself then all the suppliers and shops mentioned above are great resources, but unfortunately if you're the Propmaster, you might not have time to do much of the fun stuff. Your ability to find and hire good, reliable creative people who do good work will become hugely important.
Lots of plastic bins in a truck with lots of shelves. Label your bins with character names if they're that character's personal props. Label the specific stuff with scene numbers and descriptions. "Scene 32 - Dead Guy's ID" Things like that. You should have at least one of those rolling Rubbermaid carts to bring to set with all the day's gear, supplies, tools, expendables, etc... Rig your cart with a power strip so you can power a small lamp and keep your screw gun battery in the charger.
Make sure you always get scripts and revisions and as soon as they're available. Take notes at the meetings and make detailed breakdowns of what you need and what you have to. You can never be too careful that you're not missing something or that you fully understand what the production wants from you. Take photos of items at the rental house for director approval, then put what gets approved on hold, get the stuff you need fabricated going as soon as possible. Have everything ready for the day it shoots, but be prepared for a sudden schedule change that throws your most complicated prop in front of the camera first.
Make sure you have loads of expendables. A good collection of various kinds of tape (cloth, grip, duct, paper, double-stick). A screw gun with backup batteries and a box of assorted drywall screws, batteries, pull-ties, velcro, Sharpies, Windex and various cleaners and a few bags of rags/cloth diapers, gloves, sunscreen, earplugs if you're doing pyro or guns. A staple gun, Spray 77, dulling spray, a set of various colors of acrylic paint, and an assortment of "grunge" paints, powders, "streaks and tips" type stuff to make things look dirty if necessary. Also a small kit of electronic tools like a soldering iron and solder plus a solder remover, rolls of hook-up wire, electrical tape, and something I've found handy many times... some of those little high-powered LED keychain lights in assorted colors. Depending on what kind of production you're working on, you might want to add a little light or color to something. A soft green or blue glow inside a certain prop can imply a lot - a radioactive glow, nightvision or a monitor, etc...
If you're doing firearms, make sure you know all about the permits involved, and all of the safety stuff you need to know. And know EXACTLY what you'll be needing those firearms to do on set, etc... A good lock-box or cage for your truck or storage area is essential because you will discover very quickly how fast guns will disappear and how desperate and eager extras (and just about everyone else) are to play "gangsta" with them. A serious briefing with your actors about gun safety is super important. Threats of barring them from the set if they're caught goofing around with their props might work. If you have lots of guns and lots of extras, it's often a good idea to get their drivers licenses in exchange for the guns. They only get 'em back if they return everything.
That's really just a few of the things you might need... Depending on the budget of course. And depending on what kind of a project it is. If you're doing something low-budget you could get away with a stripped-down kit, but you should always think about lots of backup with the supplies. Things like tape rolls, batteries, screws and a screw gun and chunks of scrap wood. Things like that. For instance if you need to make some kind of contraption in a hurry that can hold a prop still in the foreground or lift something up into the frame, you'll be the hero because right there on your cart, you have your bin of scrap wood, your screw gun, drywall screws, black cloth tape and dulling spray. A few "saves" like that and you'll be the director's best friend.
And again, depending on the budget and type of project, your responsibilities as a Propmaster might vary... You might be in charge of all of the director chairs on set. Bringing them, setting them up near video village, dragging them to the next location and then storing them on your truck. You might be responsible for things that seem like they'd be set dressing or wardrobe. You'll certainly be working with those folks.
You also have to be the "dealer" sometimes... Like, if the main actor is a smoker, it might seem like a bad thing, but keeping a carton of his/her favorite smokes on your truck might keep you on the good side of an important relationship. Working with actors and their props can be difficult. Some actors are great with props and seem to have a natural ability to deal with them (I think it's called "acting"), others, however, will surprise you with their sudden inability to operate everyday things like a letter opener or a camera. You may have many surreal experiences teaching actors how to hold a cell phone or how to open a laptop.
Okay, I'll stop here.
When you read the script and imagine everything that can possibly go awry on set, you'll get a better idea of what you'll need. Talk with the other folks on the crew who might have more experience. Learning how it all works is a huge part of the fun.
