Where should I start?
From your post, I am guessing you are fairly new to electronics - and, in that case, start with circuits that run on 12V or less.
As has been said above, there are several very important considerations if you are looking at equipment operating on mains power which include safety to both person and property, insurance and whether connected equipment is compliant with any regulations.
Mains is unforgiving when it comes to carelessness or poor design. Don't even try to step up to the plate in the big league if you don't know how to hold the bat. Don't think you have 3 strikes - 1 strike is enough to take you out.
This is not to discourage you from having a go - just tackle challenges appropriate to your level of skill.