You need office furniture – cheap office furniture.  You need cheap but not cheap-looking, and you need it, now!  Cheap office furniture is not hard to find.  It can be found online, used furniture outlets, auctions, surplus stores, at garage sales, and in the dumpster (but please look in a dumpster as your absolute last resort).
Just because you’ve spent the lion’s share of your money on your new product’s research and development and you’ve been working out of the spare office space at your brother’s business,  doesn’t mean you can’t find cheap office furniture that’s comfortable, efficient, and looks professional for your office. With a little creativity and some research it can be done.
Start with what you need right away, probably a desk.  A wrap around dark mahogany desk unit with matching credenza, armoire, and bookcase is really nice – but hardly cheap. 
Think about going to an office furniture surplus store where you can find office desks that are made well and look nice.  You’ll find metal, wood and sometimes a combination and in as many styles as you can think of. These types of stores always have sales going on in their showrooms and usually an area with slightly used pieces, as well.  And look for desks with multiple drawers, both shallow and deep, as this will double for storage.
If you need a chair, a filing cabinet, plus a desk: go to a furniture discount store and look at what they offer in terms of office sets, so you can get a complete look.  And glean the newspaper for business’s that are moving or going out of business.  They may already have matching sets and probably more than one if they had multiple offices.
And get creative. For example, you could use an old door placed on top of filing cabinets as a desk.  I know of someone who got very creative and did just that.  
She started by painting four gray metal filing cabinets and placed an old kitchen door that had some unusual carvings from a garage sale, and then placed one inch glass on top of that. This gave her a good sized working space plus with the filing cabinets underneath as “legs” it saved her some valuable office space. It was unique and didn’t cost her both arms and one leg! Plus it was a great conversation starter with clients.
Get creative; go to sales, discount stores, and outlets. And remember the dumpster is your last resort for cheap office furniture. Good luck!