Unless you simply hate picking up a tool or two and doing your own maintenance there are few excuses for doing it wrong these days with all of the YouTube videos out there, Look for the one that matches the type of shifters on your bike, example: Gripshift gear cable replacement (the most difficult one to replace) My bar end shifter cables are much easier and take about 10 minutes max.
The two websites that are most often cited for elementary maintenance are the Park Tool site already mentioned by Maelochs and the Sheldon Brown website https://www.sheldonbrown.com/ Both will teach you how to adjust the limit screws on derailleurs so the chain doesn't get wedged between the frame and the wheel or go off the chainrings (your "1,2,3 gears"). Buy a cable and fix it yourself as that will a lot less expensive and will get it fixed right away versus taking the bike to a shop, leaving it for a time, and returning to pick it up. You can buy expensive cables but in my experience the standard stainless steel gear cables are just fine since I usually get around 3K miles between replacements. If you intend to ride a lot and replace cables over the years it pays to buy them in lots of 10 for under $10 with free shipping on eBay https://www.ebay.com/itm/402262687206 versus buying just one for $6. No bike shop can match the online prices.