There are plenty of free online guitar lessons around, particularly on YouTube, however unless you know exactly what you’re looking to learn - something only more advanced players are likely to know – you’d be better off using a more structured system to progress you at the right pace without any gaps or giant leaps.
The great news is that most of the best online guitar lessons services we recommend here offer a free trial of anywhere from 7 days up to a full month, so you can try a few for size before you commit to the right one for you. Most are easy to cancel too, so if you really don't get on with a particular service, you're no worse off.