Buy a book on bike maintenance (search for recommendations).
Buy a cheap bike (£5 on
eBay, anything will do).
Buy a cheap toolkit like the Lidl ones (£20).
Take cheap bike to bits.
Look at the bits you've disassembled. Refer to maintenance book for explanations and diagrams.
Put it all back together after cleaning, lubing, fixing.
Sell bike on eBay (for a minimum of £5).
Repeat with another bike of a different style and/or different components.
Occasionally buy a specialist tool and slowly build up a collection.
Slowly build up knowledge of how stuff works and how to disassemble/reassemble stuff.