Something like muffin-man’s option with a cable straight from your phone will do the trick, I had a similar set up for my son when I gave him a set of old Technics speakers, worked fine, he now has it connected to the old Technics amp which does sound much better but for about thirty quid you can’t grumble.
You could always buy a second hand amp off gumtree/ebay, some nice older stuff going very cheap, either connect it directly with a cable or via an airport express, which will make it sound much nicer as you’ll be using the airport express’s dac rather than the phone’s (check out Ken Rockwell airport express review (although I know many consider him a bit of a bell end)).
Depends how far you want to go sound quality wise.