The following information below can be used to find out how to obtain certain ranks.
When you first begin the game, you must create a Roleplay(RP) or multiple RPs to obtain level 10.
Soul: Obtain level 10 and then walk up to a Hollow Non Playable Character(NPC) and allow it to kill you.
Shinigami: As a soul in the caved area after you die, turn to the left and you will enter the Rukongai District. Once here you need to find a Shinigami and Role play with them(No, you cannot already know about them.), and then find a Captain, which the shinigami you found should bring you to. Once with a Captain you will be made a Shinigami Trainee and will have to Role Play with either that Captain, or a Trainee Squad Captain. After that you will be asked to "Form your Zanpakuto" which wont exactly happen, you just need to Role Play it out, to approximately a paragraph or more. Once done you will be made a Shinigami and then will be free to join a squad by asking a Captain of said squad to invite you.
Hollow: As a soul in the caved area after you die, turn to the right and you will enter the outskirts of Hueco Mundo. Don't worry about the hollows, they can't kill you, but it would be a good idea to gain some quick Experience(XP) before heading towards Las Noches(Refer to the Location Guide for directions). Once in Las Noches find an Arrancar(Once again, you dont know what or who they are, or where you are for that matter, but they are easy to find. They look like Shinigami with masks.) And ask them where you are, if you are lucky and either the Aizen Figure or a Member of the Espada are online, then it will be quick for you to become a Hollow. Otherwise, sit around and either kill hollows or RP to gain XP. If you happen to meet an Espada they will probably ask you a few questions and then RP turning you into a hollow. Its not required to RP back becoming a hollow, but it can get you more XP and RP XP.
Quincy: As a human at level 15 and above, Roleplay with the Quincy Elder and they will decide whether or not to allow you to become a Quincy.
Sado/Inoue: Reach Level 15 as a human and you will be given an option to become either of these.
Arrancar: Once you are a hollow, obtain level 40+ and prove yourself a good RP-er to the Aizen figure. He will decide eventually whether or not to grant you this race. If he does, then you will be quite a bit stronger and gain a Zanpakuto as well as a Ressureccion once you reach level 50.
Vaizard: Once you are a Shinigami, preferably learned all of your Kidos, travel to Earth IC(In Character)ly. Once there travel around, and if you are lucky, somehow meet the Shinji Figure. They may or may NOT decide on whether to Hollowify you. If he does not, it is either because they chose not too, or because they simply weren't impressed with your RP Level or skills.