This is all very exciting
@Sterix - and it is great that you are treating it as a learning curve and not being “precious” and protective, which (IMHO) is EXACTLY the right thing to do
In very basic terms there are 2 types of royalties - “performance” royalties owed if your music is performed live, played in open spaces (shops etc.), played on the TV, or broadcast on the radio
Then there is the “mechanical” royalty owed from recordings - downloads, CD sales etc.
There is also clearly a massive overlap between the 2
I think the key part of your question is:
But I do want to protect myself and make sure it's not going to be a case of me paying out more money than I'll get out of it (money's tight and I don't hold the purse strings in my household
In the UK you would need to join an organisation called the Performing Rights Society (PRS for Music), as a writer, which is a one off fee of £100 and covers the performance royalties
You would also need to join the MCPS as a writer to protect (collect and pay you) the “mechanical” royalties, which is an additional £100
So a “one off” £200 investment
To be brutally honest - it is unlikely you would recoup this in royalties any time soon - but it may be worth doing to cement your status and have an opportunity to frame that first royalty cheque for a few pence! - and it is a one off cost so doing it now means it will last and be in place for the future
Other countries have different organisations - but the PRS has a strong global network
I would suggest having a look on their website and perhaps contact them for some guidance