Before you start joining Ranked games, I suggest you read up on what you're about to get yourself into first.
I. What is a ranked game?
A ranked game is a draft mode game type available to level 30 summoners with at least 14 champions available. Summoners are placed on a game ladder and ranked against one another according to a publicly displayed ELO rating. Ranked games are considered the competitive alternative to normal games.
II. Premade teams
Ranked games are playable in both the 3v3 and 5v5 bracket. In the 3v3 bracket, you must queue as a complete team consisting of 3 players. In the 5v5 bracket, you can queue as a full team consisting of 5 players, as a duo queue with 1 other person, or as a solo queue. Statistics are recorded for ranked games by the type of queue that you initiated the game with.
Premade: In team-based ranked play, a player will have to set up a 3-man and 5-man team with which to play for 3v3 and 5v5 matches respectively. Once persistent teams are added, you will be required to be a member of a team to participate.
Non-Premade: In non-team ranked play, a player will be able to either play solo, or in a two-man team. This is only available for 5v5 matches.
III. Differences comparing with normal games
Ranked games differ from normal games in the following ways:
Banning Phase: At the start of every Champion Selection, team captains have the opportunity to ban a total of 4 champions. One team has the first and last ban while the other team has the middle two bans. During the banning phase, each team can see their opponent's available champions in order to ban viable options.
Champion Selection: Whichever team does not get the first ban gets the first pick. After the initial pick, teams alternate and pick two at a time until each champion has been selected. No champion can be selected more than once in a game.
Trade: When all the members of a team have locked in, there is an opportunity to make a trade within your team. Both members of the team must own both champions involved. One player can initate the trade, and when the other confirms it their champions are swapped. It is possible to execute multiple trades before the game begins. Therefore, trades are often implemented in order to secure high tier champions early on to be swapped with the intended player instead of risking the other team taking them.
Leaves: Queue dodging and/or leaving during the middle of a game results in a recorded leave similar to normal games. However, leaving or queue dodging also results in a drop in Elo (see Elo rating system) by how many points would have been lost if the game had been a defeat.
Also, read snarlmkiv's insights on ranked games here.