Meditation is the process of focusing on a particular point. You can start with a simple technique to focus on your breath or just be aware of how it's moving, how it's flowing, the pace of it, and the sensation. Try with 1 minute and then gradually increase the time, and then go deeper into subtle things to be aware of.
Slowly Slowly go through the process

For the knee pain, you need to strengthen the supporting muscles.
Simple ankle stretches, ankle rotation,
Vertcle knee stretches can help in easing the pain.

No, you have to find the root cause for it work on the cause then it will go,

Mostly the reason for back pain is stiff mucles of the back for that you need to move them, flex them, strengthen then then the back pain will be gone,

Sometimes the cause can be related to vertebral column, so its best to consult dr. For the exact cause and then work on it accordingly.

Simple stretched like cat and cow, lumber stretches, bridge pose are very good for back health, we should add these poses or stretches in our daily workout session.

Meditation in simple word is focusing at one point, or awareness at any one point, the point can be outer object (and object, point, lamp) or inner subtle things like breath, chakra, enegy...

The goal is to maintain that awareness for a very long period without breaking it.

For this patanjali say when you focus or be aware of any object its dharna, in dharna the flow of awareness will break but when the flow of awareness on a particular point become continous for longer period it becomes dhyana that is meditation...

Continous Flow of awareness is mediation...