New Champions

First, you need to design abilities. You don't need to make damage or do any numbers right away, as long as the kit seems reasonable to you at first and you can identify both strength and weaknesses and level them out as you test the numbers later.

Second, you must recognize a character/champion within your kit - don't just think of a picture and make abilities off of it (but that can work, too.) Mess around with their design and make a lore accordingly.

Finally, you will need to post it on to the Skin & Champion Concepts section in the LoL Boards which are here.

It's no guarentee your champion concept will be recognized right away, so give it time and commit to updating it often, as well as take the advice of other people regarding anything they advise you on.

Hope you enjoy making a champion :)