My baby is only 5.8 kgs weight at 6 months old. She is on breast feed only. What should I feed her now?

Depends. Breast milk is the best nourishment available for infants. However, not every mom produce enough milk to adequately meet the need of infant. I wonder if your baby was born prematurely with low birth weight. If she weighed 7 lbs (3.18 kg) or more at birth, current weight is certainly low for her age. If so, she needs formula supplementation beside adding cereal to boost calory intake.
Depends. 5.8k places your girl on the low end of us growth charts and if she is maintaining that rate or has improved over time breast alone can provide great nutrition if your situation allows. The addition of simple cereals made from dry powder mixed with your milk is a good starter food. If weight gain has slowed over the last months be sure to have a well check to no other issues need to be addressed.