Correction:  Student's sentence   Corrected sentence        Grammar explanation       Better sentence            

These days, I can see a lot of people who do exercise.  

These days, I can see a lot of people who exercise.  

You can just remove the do. Or you can have do exercising. 

I'm  also exercising now, the reason I exercised in the first place was just to lose weight.

I'm also exercising now, the reason I exercised in the first place was just to lose weight. 

Please be careful with your spacing. The rest of the sentence is correct. 

I'm still on a diet, and now I want to take a 'Body Profile.

Correct sentence. Just with the punctuation maybe. 'Body Profile'

I think the reasons why other people exercise are similar.

This sentence is correct. 

I think the people start exercising to lose weight, After dieting and building a body, to do exercise to increase their's strength and take a body profile.

I think people start exercising to lose weight, after dieting and building a body, to do exercise to increase their strength and take a body profile.

After - after - the proper use of capital letters

the people - people

their's - their