A good set of Allen keys will go a long way to basic maintenence.
Then if you want to replace the chain and cassette, could add a chain whip, chain splitter and a cassette lock ring tool.
Other jobs might only need to be done occasionally, such as BB replacement, pedal replacement etc. So it's a dilemma whether to buy an expensive tool kit up front or just wait until you need to do a specific job and buy the tools when you need them.
House hold tools like pliers, hammers, wire cutters always handy to have.
Good luck