How I can purify my physical body?

If one is angry and resentful of one's own body, that is a good place to start. No matter what body one has, it is a gift of opportunity.

SN56.48:1.1: “Mendicants, suppose the earth was entirely covered with water. And a person threw a yoke with a single hole into it. The east wind wafts it west; the west wind wafts it east; the north wind wafts it south; and the south wind wafts it north. And there was a one-eyed turtle who popped up once every hundred years. What do you think, mendicants? Would that one-eyed turtle, popping up once every hundred years, still poke its neck through the hole in that yoke?”

SN56.48:1.7: “It’s unlikely, sir.”

A human life is a rare gift:

SN56.48:2.1: “That’s how unlikely it is to get reborn as a human being.

Knowing how precious human life is, we might feel guilty about resenting our body. We might feel guilty of wanting a "better body for a better opportunity". And then we might abuse ourselves in some way. And then all would spiral down unskillfully into despair.

Or we might simply let go of resentment and start afresh. Even if we resent our own bodies, we can start afresh:

AN5.161:1.3: You should develop love for a person you resent.

Letting go of resentment, we can hug ourselves and celebrate sweating a little more than yesterday, walking a little more than yesterday, trying a little harder than yesterday.

Is this easy? No, it is not easy to give up resentment. I know it is not easy because I am losing my eyesight. And in letting go of my own resentment at loss of sight, I have discovered a brighter world filled with the generosity and kindness of others. Resentment never helped me, so I had to let it go.

Please do not give up hope. Give up resentment. Start by giving yourself a well-deserved hug of acceptance. Then slowly, gently and kindly, lift yourself up and take care of this one body that you have. Use this very short, precious life that you have to be kind to yourself and others. Exercise a little, then a little more. Cheer yourself and others onward with your efforts. Give up resentment--it is useless.

And when you give up resentment, guilt evaporates. Give up resentment and focus on the goodness inside of you:

AN5.162:8.1: In the same way, at that time you should focus on that person’s pure behavior by way of body and speech, and on the fact that they get an openness and clarity of heart from time to time.

Give up resentment, open and clarify your heart. Focus on "that person's pure behavior". That person is you. There would not be guilt without a knowledge of pure behavior. Give up resentment. Extend love and kindness to yourself and others.

The first step in the Noble Eightfold Path is simply Right View. Give up resentment and choose loving kindness to yourself and others.

Dhammapada;

109(?) If one maintains respect for worthy people four things increase; health, beauty, happiness, vigor ... Sloth corrupts physical beauty... (chapter 18)

I also heard that once Buddha said that his monks look as they are similar to flowers that are well rooted because they (monks) were nourished by the mindfulness immersed in the body. No reference for this.

Skin problems can have many causes like dietary ie and it's probably not due to attacking with hand which nevertheless is an obstacle to states that are good, heavenly rebirth and ariya stages.

Body is a disintegrating ground for disease, quickly worn out, if one wastes time and is nourishing it for no good reason then it is very regretful.

One is purified by good actions basicly.

Also I masterbate a lot which is the cause of my weak and ugly body. I don't even have the strength to exercise the way I want.

I think you've already identified the crux of the matter. You've been directing most of your energy into a frivolous un-productive activity and have nothing left for other productive ones. So, the next time there's an urge, get outdoor and start doing some cardio exercises. There's nothing better for your body than doing cardios out in the open air, running, jogging, biking, walking, etc. Since your body is still weak, don't overdo it. Start out light, and build up the strength and stamina gradually. Combining exercises with adequate sleep and a healthy nutritious diet, and also nurturing your spiritual side with meditation and Dhamma cultivation, you should be on your way back to a strong healthy body and mind in a few months.