Hi Dave and welcome to the forum...
By and large you aim to rest the ladder below the sill, on the sill can cause damage, and on UPVC it is also very easy for it to slip sideways on you too.
Andthe same can go for resting your ladders on gutters too, it really isn't to be recommended, if you can find a spot without gutters, or extend the ladder higher and lean it against the side of the house so that as you climb up you only have to step directly onto the flat roof, though that too has it's inherent risks!
Read the info on t he link that Roy has put up there, you will learn a lot from that.
Roger as usual [keep reading posts on this forum and you will see just what I mean] has to take a dig at WFP by making an incorrect statement.
Until he mentioned it, it was just a post about the correct way to use a ladder!
You will I've no doubt have by now read many of the posts on here about WFP.
Take in what you read and save it up for later, but for now learn your craft the traditional way, it gives you by far the best grounding in window cleaning.
But do consider upgrading to WFP at some point in the future...because it IS the FUTURE!
Try and find someone to show you one to one how to use a ladder safely, how to extend it and lower it.
Newbies can cling to them like limpets and gibber with fear when they need to bounce it off the wall slightly so they can lift and lower the extension!
But keep asking your questions on the forum, some will keep telling you to go WFP but others will also do their level best to answer the questions you pose.
Squeaky is our resident, rabid WFP hater and will always manage to introduce a certain level of incomprehension into the WFP debate....and even when there isn't a debate he'll always amuse us in some manner with his flat earth mentality on the merits [or lack of them in Squeaks opinion] on WFP;D
Us moderators [myself and Tosh] would not dream of fanning the flames of his obsession....we leave that to the other forum members
Enjoy the forum, we all hope we can help in any way we can
Ian