I co-founded a start-up (been on companies house UK for about 1.5 years). Not my first job. In this post I not only want to tell you what my day-to-day includes (at the end) but also what it means to work in a start-up with any tips I can remember. In addition to being a dev I'm also the Company Secretary - which is a job itself.
Honestly, I make this parallel a lot - being a part of a start up is just like relationships and sometimes more specifically sex. My general point is that no-one really has any idea what they are doing. Every start-up is different, just like every partner is. Each requiring a different level of attention, needs, affection, the list goes on. You're friends/colleagues will try and tell you how you should do it, based on their experiences and ideas. But overall, you're the one who has to make the decisions. You can take their advice, which is usually not terrible and use it to come to your own conclusions - like I said, every start-up is different, generalities (as well as other methods) won't always work. All I'm saying is be careful and not "I read this thing once, so it must be true for me, for my needs.". Proper analysis and deriving accurate and useful conclusions is half the battle of anything we do, not just start-ups.
You will be wrong. Accept it, but not afraid of it. In life everyone makes mistakes, but in a start-up you'll make more mistakes than ever before. Why? Because you'll be under more pressure, more work load, and more drive and passion than any other job.
My Short Story:
When I first started my ventures in a start-up, which I am a co-founder of and started through an "incubator" I was drastically different to what I was previously. When this all began I became quite afraid of saying the wrong thing to both my partners and to any potential clients/anyone in the outside world. This fear was manifested by wanting to succeed. I felt that if I said the incorrect thing then, not only myself, but the company would be looked on as incompetent.
Only after a year or so did I actually realise what I was doing and so stopped. It was stupid of me to think like that. But it happened and I improved myself.
Look after your home life. A start-up is not a job, it's a lifestyle. I started this venture single, and then half way through became heavily involved with my fine lady, engaged. We have had a few discussions about too much work etc. So be careful and ensure that the ones you love don't suffer because of work commitments. Find that balance.
It's not guaranteed. As with any startup there is risk- a lot of it too. It's your job to figure out what risks to take and which to pass on. A job in itself almost. So be prepared for the possibility of the company dissolving and leaving you with no return apart from one hell of a bumped up CV and a shit load of experience to go with it!
You will not be prepared for everything.
"I don't know what I don't know."
Is a statement that holds true a lot of the time in a start-up. And the things you do know you need to know you have no time to learn. So it's never a single person venture. No matter how much research you do or however many books and tl;dr's you've read. There's always more to learn. Always remember that you can't beat yourself if you made a mistake and didn't know something which would've helped. It happens. It will always happen.
Honestly, this varies so much it can't be said simply. I will however give you a run down of all the things I've dealt with over the past year: Investment documentation/legal documents, hiring developers, hiring processes, dealing with recruiters (can be incredibly frustrating), dealing with Company Secretary responsibilities and duties. Looking after an entire project, of my own creation, managing people, managing customer and clients, ensuring a good atmosphere and that junior developers feel good about working for me. SEO, and a little bit of company website management. The next one is a big one for me, mainly because it's what I'm best at - problem solving - can be any problem I'll find a way to solve it. Managing company best practises and style guides, dealing with complaints or arguments, disciplinary stuff. Looking after roadmaps and thinking long term as well as short term. Blogs, content marketing, any research that is required for anything that might come up. Going to events and expo's - pitching the company and the products/solutions on offer. Server configuration/setup/cloud management, cloud networks & security. General debugging from F12 to wireshark. Customer network configuration and customer products as well as anything else my company integrates into/with. Taking out rubbish, moving office, cleaning/DYI.
I think that covers most of it. My skill and experiences in the areas mentioned above vary but it is an extensive list so you can see why and I'm sure there'll be some I've missed off there - oh yes, I've missed off all the numerous languages, frameworks/methods that I've used and then those I've assessed for adding a return to the company.
I hope this gave you at least some insight into, at least what I get up to in my company. I think this covers most of my thoughts. If you have any further questions @ me in a comment. :)