A set of guitar strings should cost around £6 ($9) and there are good discounts for buying in bulk. HOWEVER playing with strings that are too thick (aka 'heavy') leads to high action and sore fingers. Here are my tips for finding the right strings for you.
I receive nothing for endorsing these strings or brands. These are what I use and have found massive benefits to my private students when they try thinner strings
Action is the height of the strings from the fretboard. At the 12 fret, this should typically be around 6/64th of an inch (2.38 millimeters) If its way off this- thinner strings is the easiest and most affordable way to help this.
Gauge is the thickness of string. The thicker the string, the more bassier the sound, but also the harder it is to play. This article and video is written to help beginners make an informed decision about what strings they buy.
Super thin Acoustic guitar strings (Bronze in colour)
Standard steel string acoustic guitars come with Gauge .11 or .12 gauge strings. If your fingers seriously hurt, try a lighter string! Acoustic guitar strings go as light as 0.09 (or 9s for short) but try 10s or 11s and see which feels right for you. Personally, I use Elixir gauge 12s.
Super thin Electric guitar strings (Silver in colour)
Electric guitar strings usually come with 10 gauge strings. These are usually fine, unless your action is massively high OR your a beginner learning string bending. If either of these are true- 9s or even 8s can really help. Personally, I use D'addario gauge 9s.
REMEMBER: Everything here is written as a guide so you can make an informed decision for yourself. Experiment with a few different strings and gauges and find the right one for you. When you've found then- buy in bulk for big discounts! (and try eBay..)