IF YOU ONLY BUY ONE THING… Resistance band
Resembling giant elastic bands, these are light and small, but can be as effective as using weights. Find them in fitness stores, or try Dyna-band (dynaband.co.uk).
Try this: Put your right foot on one end of the band (and hold the other end in your right hand). Exhale and slowly raise your arm out in front of you, then extend it upwards into the air. Inhale and then return to the starting position. Perform 15-20 reps with each arm, then repeat three times.
Resistance bands are affordable and can be used anywhere