To get started with your website editing please watch the relevant videos for your website.
I recommend you watch them all in order, it won't take long and you will learn many useful tips which will save you time in the long run!
There are two sets of videos. Those for Concrete 5.6 (pre 2016) and concrete 5.7+ (Autumn 2016 onwards), if in doubt ask us and we can tell you where to look. There is also a page of client specific instructions where additional videos will be and anything directly relevant to your website, so it is best to go there (the relevant concrete 5 videos will be listed there also).