How to teach my one month rottweiler puppy to pee in a specific place

Do you mean inside? That will take a lot of work and nearly constant attention. Every time, and I mean EVERY time it starts to go you need to move it to the place. Praise it when it goes in the right spot whether it went on its own or had to be carried. You can see why starting immediately is essential, before it gets too large. This technique will work in the yard or garden as well, but will take more time.

But since it's too small I am keeping it inside home.. actually its one month old when should I leave it outside home..?

You should not leave it outside the home for more than a short time ever, and then inside a fence. At one month it should be barely weaned, normally puppies should be with their mother until about 6-8 weeks of age.

Beardedsumo thank you for your answers are very helpful since it is the first time for me having a dog.And during this period I am just taking a lot of information. I am keeping him inside since is cold outside during this period and only during the lunch time I am taking him outside in the garden to play..

Why was he separated from his mother at some a young age? This is both dangerous and foolish. Is this something the owner of the mother recommended?

Yes he decided so... its was impossible for him to keep 6 puppies in a small flat so he had to give them for adoption.. and I adopted .

Wow, 4 weeks is WAY too young to leave that pack. Hopefully this isn't your fist pup. Chances are you are going to have a lot more issues than just house breaking. I wouldn't be taking him outside until he is up to date with parvo vaccines. You are going to have to socialize this pup as soon as you can. Puppies learn the most from their litter mates and mother in the first 8 weeks, since he has been taken away from this crucial learning environment, you are going to have to pick up the slack.

At 4 weeks you can't expect much training. He is a baby, think new born stage. The best you can do is have pee pads/ potty patches in several locations. This is a BIG breed so as soon as he is up to date with parvo vaccines, you need to break the habit of urinating and pooping in the house. And this is a HARD habit to break. Get him vaccinated ASAP.

Consistency is key here. Every time the puppy eats or drinks you take him to the designated potty area (be this outside or indoors), you praise and offer a small reward for using the potty patch (I really recommend the patch because it is like a little turf of grass, easier transition for when you start house breaking him).

The biggest thing I can stress here is at 4 weeks your dog is not vaccinated, Parvo is running rampant and with the warmer weather upon us you are going to be at a higher risk. Keep your pup inside until he is vaccinated. Go to the Library and get some books on puppies. Get some socializing books are read every bit of info you can get your hands on.