I've started weight training and I'm in my 4th week now. I'm seeing big improvements. Now I want to start tracking my workouts & diet. Is there a software or a website which allows me to do that?
I use a variety of apps.
Fitocracy is good for tracking workouts. It's gameified (you get points and levels for your workouts, and there are achievements) which can be fun and motivating.
MyFitnessPal is good for tracking your diet. It has a large database of nutritional info of food, so the hardest part is recording an accurate portion size.
If you do a lot of running/cycling/rowing, then Runkeeper may also be useful. It plugs in to Fitocracy and MyFitnessPal nicely.
I made Evertrainer.com as a lightweight program builder/tracker. Might be a good solution for your workout tracking.
Here's quick gif of it, to give you a sense of it: