What a healthy weight for 21 year old girl?

It would depend more on your height than age. There are online calculators that can give you a range of "appropriate" weights for your height.

But unless someone assessing your health knows your overall health, body build, per cent body fat vs. muscle, etc , a "healthy" weight is what YOU are healthiest at. Weight ranges are only tools to give health professional a starting point re: your weight, to include in their overall evaluation.

Do some googling for "healthy weight range calculators", but don't obsess about it if you're outside the range. Talk to your doctor if you aren't sure.

Is doing just push ups bad for overall muscle balance?

I doing just push ups would probably result in some muscle imbalance, so you should really try and find a way to do some pull ups. There are other back exercises such as supermans, which can be done with just bodyweight, and you can do shrugs and shoulder

How to lose 19 kg in 2 months

Slimming is a subjective matter as it depends upon person to person. Maybe you are 50 Kg and wanted to lose some 5 kg only at the same time someone with 80 kg targets to lose some 10 kgs in a month.

Why do people practice yoga?

Many Intellectual out there have so many things to say but being a YOGA STUDENT I can tell u my experience with yoga.Back in the days I used to get some criticism for being skinny and weak. So it was my 11th grade and I chose Physical Education as my optional subject. And starting