I'm trying to get it but I don't know what probabilities are to catch it, or where should I have to search for it.
Find a route where you can enter and leave the route with nearby grass. (Ecruteak City is a good start since by the south side of this city has nearby grass). Take about 20 steps in the grass with the repel. If no Pokemon appear, Raikou or Entei are not there leave the area and come back.
Once you find it put it to sleep, if you try to trap it it will just roar you away. If you want to avoid other random encounters, you could use something like a level 39 haunter/gengar or jynx with a sleep move first in your team, and use repels so that everything below that level will stay away from you. It will be easier to find after you've found it once. Another way is to use a golbat, weaken it, and use mean look.
I would recommend the Master Ball for catching him. If you don't want to waste a Master Ball on him, you can use the Pokemon Cloning cheat. Besides cloning the Pokemon, it will clone the item attached to the pokemon. Just attach the Master Ball to the cloned Pokemon and you will get two.
Then you can use the Repel and route switching technique to hopefully find Raikou. Good luck!
Once you've seen it once, you can track it by looking in the Pokedex entry as it's a roaming legendary, although every time you fly to a city it will have moved somewhere else. And you can paralyze it, but it would require the use of a pokemon that knows stun spore, or the repeated use of thunder wave, which might not work. Just put it to sleep and but don't use the masterball, as you should use that on Ho-oh or Lugia.