A lot depends on your volume. If you do 10,000 transactions per day it is much different that 3 sales per month.
For an early startup with perhaps 10-20 transactions per month a plain old-fashioned ledger book would work fine. Perhaps a spreadsheet would work as well.
At some point you are going to start to grow and you will quickly outgrow such a system. At that point you most likely will want software specifically designed for accounting. Many small firms use Quickbooks, but there are other options as well.
As you grow you will most likely need to work with am accountant, particularly for preparing your year end statements and filing taxes. It would be a big mistake to select a method or software package that your accountant was not comfortable with.
For the initial start, using Excel is a good idea. Also, make sure that you can convert the same to a CSV file, which these days is used by almost all the top accounting softwares and banks to provide access to their data. You can also store it as a XML file when using Excel 2007 and above (may be possible with oldeer versions, but not experimented there).
If you keep in CSV or XML format, later when the total transactions grow, and you have enough to invest in an accounting tool, you can export/import the same without any BIG trouble or hassles.
Best way is to keep a WorkSheet for Income and another one for Expenses, then using Excel VBA or Formula, you can manipulate the data and work out the same. A good example of an individual budget, which can be adapted for a startup is:
http://www.simpleplanning.net/Financial%20Calculators/BudgetPlanner.html?gclid=COHnhIiNva4CFWZKpgod0wOr5Q https://www.moneysmart.gov.au/tools-and-resources/calculators-and-tools/budget-planner You can easily download and enhance/modify the same to adapt to your needs. These will help later also, as these are set formats whioch many of the online/commercial tools can also understand and make use of.
I would recommend looking into using accounting software. Sure, you may not have many transactions right now while in the early stages of your business, but it is important to maintain organized financial information from the very beginning (so as to avoid headaches and extra work in the future). Excel may work fine now, but eventually, and especially to accomodate for business growth, you are going to want to use something more robust.
I recommend QuickBooks Online Simple Start. You can easily track your expenses and revenues, and even pay your business bills through the software. QuickBooks Online Simple Start even gives you the ability to import your existing Excel data. You can try out a free trial for 30 days, after that it is only $12.95/mth - a pretty good price to pay for organized finances! Best of luck.