We've been running a retail clothing store since 1993 using software from Vigilant.
http://www.vigilant.com
They are a Canadian company, but for most things that does not matter.
When we started, I looked around and found several POS systems that were based on general-purpose accounting packages with added modules to provide POS functions. In most cases, by the time all the components were put together, the software was over $3000. On the other hand, Vigilant was a DOS-based package for $900 that already had most functions built in. It is very compact and fast. Since that time we upgraded to their last release for DOS before they re-wrote everything for Windows, which is the main product they sell now.
The software is a transaction-oriented database. Each time you make a sale, for example, you enter info like stock number and quantity, and it tells you the current stock level, puts in pricing, allows for manual discount entry, adds in pre-set sales tax rates, etc. You enter as many items (separate lines) as are involved in the sale, and it updates it all as you go. When you finish that you specify how the payment is happening and commit. Only at that point does it take all of your info, open the appropriate files and post it, then close them again. So your files are never open and active until an actual transation is being posted. But every posting is done right when the sale happens, and immediately afterwards all the database is up to date, so it's a "live" database in that sense. The data in the files are generally ASCII text characters. If you choose, you can keep individualized customer records by setting up each customer as an "Account Receivable" and recording each sale as an invoice in that account's file. But there's always a default account called "CASH" where everything else goes.
The basic system includes inventory records and management, point-of-sale activities, and general accounting (Accounts Receivable, Accounts Payable and General Ledger) functions, and a few others all in one integrated database. For our store we did not buy the modules to have credit cards, etc processed by the computer. We do that with a stand-alone credit card machine (contract with a processing company), and simply enter the payment info manually in the computer at the end of a sale. Similarly, we did not choose the scanner options for bar-coded price tags - we create price tags with stock numbers by hand, and manually enter the stock numbers for items as we sell. Our small store uses only one computer, so no networking modules are needed.
The software allows you to create several completely separate "companies", each using its own complete set of data files in separated subdirectories. So you can actually run different companies on one computer, or create virtual "companies" with copied data to practice or play "what if" scenarios. The software includes the ability to set up a "Training" company with a copy of real store data files where you can train and make mistakes without lousing up your real database.
To run it you need modest computer resources, of course, and a good reliable backup system as others have said. You also need a receipt printer for sure, and the software can handle lots of models, connected to standard ports. For our start-up 14 years ago we bought a dot matrix-type printer using 3¼" wide standard cash register tape rolls and a ribbon cartidge, connected to a serial port. It also had a connector to output an electrical pulse signal to our cash drawer to make it open. (The printer does this when it receives the ASCII character "BEL", or ASC(7) from the software as part of its data stream to print.) Today you likely would choose a different printer. You might not choose to buy a cash drawer that opens on a remote signal.
If you look around there are retail trade magazines published by several firms - try finding them in public or university libraries. They always have ads in them for companies supplying software, hardware, and complete systems for POS. Most will be happy to sell you too much, so understand you immediate and possible future needs before you buy. If you are not really computer-savvy, a local supplier with support available on short notice can be quite helpful. In general, though, few typical computer stores know anything about this. You are more likely to find the help and equipment from business-supply companies or even some accounting companies.