What business subjects/skills should a first-time entrepreneur know, either at the beginning of, or as he/she is building, his/her company? Why?

Marketing & Sales. A lot of entrepreneurs are "doers of the thing" but at some point, you're forced to take the shift and become a "marketer of the thing".

Think of it this way: You can have the best product on the market, but it doesn't matter if noone knows about it.

Ofcourse, there are so many other important bits I'd love to tell you about - but, start with these and you will see traction, the rest will follow.

Hi:
Already some solid info here, but I would add two things:
1. Explore what kind of startup courses are offered within your community. These generally offer the basics in creating a business, tie you to local resources (and maybe mentors) and are reasonably priced.
2. The Business Model Canvas is a great place to start working through what you need to know to create and build a business -- it's available through strategyzer.com.
If you wish to discuss, send me a PM through Clarity for 15 free minutes.
Cheers,
Kerby

One of my favorite resources is an Ebook on Audible. Critical Business Skills for Success by The Great Courses. It's 34 hours of value. Get an audible account with your first credit or two for free. Apply one of those credits toward this book. Some of he most valuable traits one can possess as an entrepreneur is an open mind, a love of learning, and unbreakable optimism.

The most important always is SELLING. You will start on your own and will you will have to do a lot of things. But there is one that is the most important and the one that will make your business happen, and that is selling. And also is the one you can never outsource. Of course you can grow and have people in your team doing the commercial work but the entrepreneur is always selling.

What first time Entrepreneurs must know is:

1. How Money works. Especially, a good dedicated focus on cash Flow and not Booked Income.
2. Relationship Building. Do not focus on gaining customers, focus more on forging great relationships. Even if it means giving away your knowledge for free. Customers will eventually get attracted to those entrepreneurs who have their best interest and not a selfish motive to generate sales.

Picking the right Partner!
Here are the 8 qualities you should watch for while deciding on a partnership with a co-founder for your startup.
1. A good partner has the life-design that fits being in a startup (can spare the time, energy, focus and finances required)
2. A good partner shares the problem (sees the world in the same unique way)
3. A good partner has depth of knowledge on at least one topic related to the startup (you find yourself learning new things in each encounter)
4. A good partner values good execution over genius ideas (knows success is more dependent on doing hard work and the teams’ problem solving capacity rather than the ingenuity of ideas) 
 Hint: Here is a litmus test. Ask your potential partner what she thinks has made the well-known startups successful. If the answer is among the realms of “a genius idea”, or “luck” take these as warning signs.
5. A good partner focuses on increasing the quality of decisions (honestly prefers reaching better business decisions rather than ‘being right’)
6. A good partner listens to you (open to learning from you and enjoys discussing ideas even when their direct impact on the business is not apparent)
7. A good partner is intrinsically motivated (follows-up on her tasks without external reminders and volunteers for open tasks)
8. A good partner is a life-long learner (sees the big picture, values design-thinking, quick to grasp abstract concepts, can change her ideas when presented with the right data)

Listen to the podcast where I talk about these: http://commonwisdom.co.uk/8-principles-for-picking-the-right-co-founder-for-your-startup-podcast/

Well... If you're serious about generating income, any schooling will pollute your brains with layers of nonsense... you'll have to unlearn first...

College is fine, as a social experiment.

Useless for generating cash, unless you're in a course of study which requires credentials... like... rocket science... medical... etc...

Your far better off, finding a local person willing to mentor you about what they do.

Sales and marketing will always be two skills very very high in the scale of nessecity but my opinion is that due to the fact that we are experiencing the electronic/technological evolution age a new group of skills climbing up the scale and taking first places. I believe that It is necessary to all entrepreneurs to develop digital skils such as "Digital marketing" , "e-commerce" , etc.

Good skills are public speaking, leadership and coaching. Thank you.

I think every student should learn to write different types of papers well. This skill will help you to write various forms of reports at work and it will take much less time in the future. Also, with good writing skills, you can find a great high-paying job. Writing papers for money jobs is something which lots of people look into if they are not pleased with their existing occupation. They might feel like the time that they spend at work is worthless because of a lack of money or time to do more meaningful things, but if you know where to look and what you have to give, you can find a great job as an educational author.