At 1.81m and 68kgs I recommend you buy healthy foods and eat yourself bigger, start eating your body weight in lbs x 15 or 16 (2250 to 2400 kcals) and monitor your body weight weekly, if you gain less than 0.5lb per week on average then up your intake, I would suggest your train each muscle group 3 times per week and concentrate on the main compound exercises (squats, deadlift, bench press,oh press and rows/pull-ups) and aim for 6-12 rep sets and aim for 30 reps total per muscle group per workout on upper body and 40-60 on lower body exercises