Radahn can be found in the south of Caelid, more precise at the coast.
I not sure if there a other ways to get there, but I did the following (BIG spoilers ahead):
*** Spoiler - click to reveal ***
First thing to do is find Blaidd in the East of the Limgrave. You will find him sitting at the top of a ruin, howling. To get him down, you will need to use the Snap gesture. It is obtainable by talking to the merchant at the Church of Elleh after finding Blaidd.
Blaidd will task you with killing someone. Its a mini boss which is found in the south of Limgrave (Forlound Hound Evergaol). After killing him he will appear at the entrance to the Evergaol and requests that you find a friend of him, a smith named Iji.
Iji is found in the north of Lucaria (There is a teleporter in the lake to get you quicker there.) Talk to him.
Next you will need to get through the manor Iji mentioned, the one you can directly see from his location. Getting trough there allows you to talk to Renna again. She offers you to server her. Accept and find Blaidd in the Siofra River Depths. The lift to get there is found in the east of east Limgrave. Make your way to the second lift, Blaidd is found at the cliff next to one of the jumping extenders. He ask you to take advice of one of your two other allies. Do so (personally I spoke to Iji, Im not sure if its import to who of the two you talk to), and Blaidd tells you to join him at the festival Iji mentioned. This festival takes places at Redmane Castle, which you can find at the southern top of Caelid. Fight your way through the castle and after the boss battle there the festival should start (make sure beforehand that Blaidd said he will go there. If nothing happens try to pass time to night). To start the actual fight you need to talk with the annoucer and tell him you are ready. Do so and get to the beach. Now you can fight Radahn.
The fight itself is kinda unique. There are many summon signs for npcs. I would recommed to use as many as you need. More importanly summon Blaidd to continue his quest. Radahn will shoot with big magic missiles at you till someone gets near him. He has some heavy hittig attacks, which can be dodged with Torrent more easily. Be aware of his jumping attacks and his magic attacks, especally when he starts to get enraged (happens after enough damage is dealt). However besides using Torrent and the summons, there is no need for special strategies. Hit him till he is down.
If there a other ways to get to Radahn let me know in the comments so I can edit my post.
Edit: According to Chronic Killer it seems to be possible to skip this quest line if enough advancement in the story is done. Might be worth it to skip Radahn and come back after some time if you are not interested in the quest line.