buy this and check chain wear
replace if it's worn
get some chain lube and lube the chain
check the bearings, the wheel hubs, the headset, and the bottom bracket
check they're smooth, and that they have no lateral play in them.
To check the hubs, just lift a wheel off the ground and spin it. Also try wiggle it side to side gripping the rim.
to check the headset, grip the stem and a fork blade in each hand, and spin it, and try wiggle for play.
Checking the bottom bracket is difficult on that bike. You have to remove the chain.