You need a bunch of things.
- left hand shifter, or a left hand replacement brifter if your bike has integrated shifters
- front derailleur mechanism with a band-on (aka bolt-on) clamp mechanism and diameter of the same size as your seat tube
- new inner gear cable and cable housing for the front mech
- a pair of cranks that have the same length, and same size pedal mount hole as your existing pedals
- the right hand crank should have chainrings sized to suit your need. Since it takes a 50T now, smaller toothcounts should fit fine. Cranks should have the same BB interface as your current BB axle (probably square taper but others are possible)
Depending on the details you may also need
- new bottom bracket axle with more length
- bolt-on frame cable stop for the seatpost. May need to be angled, OR
- an "under-BB" cable guide. Generally it is a piece of plastic, and is screwed into a hole in your BB
- bolt on downtube cablestops or holders (if cable zip-ties aren't to your taste)
- new grips/bartape.
If you have to buy all this new, it may be cheaper to buy another bike.
If you have access to a donor bike, something that has been crash damaged could be good.
Or start exploring your local bike cooperative or ebay/CL/gumtree etc.