You have to go through the ranking process, which means people have to approve your map for rank. For your map to be ranked it must also follow the ranking criteria found here:
https://osu.ppy.sh/help/wiki/Ranking_Criteria
1) You start by having people look at your map and suggest changes. This is for other people to catch mistakes you didn't catch yourself. Best way to do that is posting in the appropriate threads here:
https://osu.ppy.sh/community/forums/60. This is also probably the most frustrating part of ranking a map because many modders (people who looks at maps and request changes) have specific preferences regarding what maps they want to see. It may happen that your map doesn't fit the tastes of any modder, in which case you'll either need to make sure it has no mistakes on your own or find someone somewhere who will provide feedback on the map.
2) The map needs to have enough hype to indicate people actually want it ranked. A beatmap needs at least 5 hype to be qualified for nomination. See -mint-'s comment regarding that.
3) After you feel confident that your map has no mistakes, follows the ranking criteria, and has enough hype, you can ask a beatmap nominator to nominate it. You need 2 nominations for a map to be qualified. A list of mania BN can be found here:
https://osu.ppy.sh/help/wiki/People/Beatmap_Nominators
4) After your map is qualified it will remain qualified for around a week or so for people to play it, take a look at it, etc to make sure it really REALLY has no mistakes. If there are mistakes, then it becomes disqualified and you will need to repeat step 3 again.
5) If your map has not been disqualified within that amount of time, then congratz, it's ranked!