There is not a real easy answer for your question. The balance sheet is a reflection of the book balance sheet...not necessarily for tax. Checking account balance obviously would not be the same for beginning and end of year. What happens in your checking account is your cash flow (as you stated). But the cash flow affects many other items.
You state you have assets of over $250,000 and income of over $250,000, or you wouldn't have to file the balance sheet.
I suggest you print off a copy of the balance sheet, and pencil in amounts on the various line items, Note the dark blue lines are not to be filled in...the light blue is for entries.
https://www.irs.gov/pub/irs-pdf/f1120s.pdf
Instructions for 1120S
https://www.irs.gov/pub/irs-pdf/i1120s.pdf
The balance sheet is the product of various things, cash, net income, distributions, etc.
You might want to consult a professional for the first time you do this...someone with accounting knowledge.
But, the simplest way is to get the asset side done first. Those numbers (especially if you don't have a lot those items) should be fairly easily obtainable. Note that this is book numbers...not necessarily tax unless book and tax are the same.
On the liabilities and SH equity side, lines 16 through 21 (if you have any) should be easy to fill in. Capital Stock and additional paid in capital should reflect the money and/or assets you contributed initially into the corporation to get your shares of stock. Treasury stock would be a very unusual item, especially for a single SH company. I'm guessing that would be zero.
That leaves retained earnings (line 24) and adjustment to shareholder's equity (line 25); as the items that will fluctuate.
Hopefully this link works...to intuit community about 1120S balance sheet questions.
https://ttlc.intuit.com/community/forums/searchpage/tab/message?advanced=false&allow_punctuation=fal...
https://ttlc.intuit.com/community/business-taxes/discussion/balance-sheets/00/1203982
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-reconcile-my-balance-sheet-in-turbota...
https://ttlc.intuit.com/community/business-taxes/discussion/where-to-report-shareholder-distribution...
Good luck...this is very difficult to explain in a forum such as this and if you don't have some accounting/bookkeeping background.
Again, for this first year, you might want to consult a professional for help.
**Disclaimer: Effort has been made to offer correct information; but due to the discussion forum limitations, the poster disclaims any legal responsibility for the accuracy of the poster's response**
