Q: How do i understand when iam able to exercise with more kgs with progressive weight training?And in how much time should i begin see some changes in me?

A: You just need to pay attention to how easily you can lift the weights you're lifting. If you're able to perform 12 reps in a set of some exercise in good form and still feel like you could do another rep or three, then it's time to up the weight for that exercise. The last few reps should be quite strenuous (which doesn't mean you contort your body to perform them, but that your muscles maybe crying for rest but you push on with perfect form!) and in general 12 is the upper limit of reps for most exercises. So if 12 reps start seeming easy or even only moderately difficult, then it's time to up the weights. It's not uncommon for your strength to increse 50% or more in just the first month or two of training!

As to your second question, as I said, strength changes should be noticeable immediately and you may well be increasing weights every workout when you're just starting out. Body composition changes will be slower (and remember, for both losing fat and building muscle the slow progress is the good, lasting progress!) but after the first month or so you should definitely start noticing changes and after one year of good diet and training you may well be a completely different person!