not to go too off topic, cause i notice i am doing it and this thread isnt about dreads lol.
but IMO, its best to start off with shorter hair to let everything work on its own from scratch. all you do is wash and dry the hair regularly. and thats it. the straighter your hair, the longer it will take. dont expect this to be a 6 month or even 1 year thing for natural dreads, especially if you've got straight hair. and dont expect it to look like black dreads, because we just dont have that natural coarseness and curl. its a long process that will naturally occur. took me months for the first one to form on the side of my head. then a bigger one in the back and so on. also, as your hair starts to dread, you will notice it start to shrink up and get shorter as the dreads slowly develope.
quick tip, if you work an office job or retail, i wouldnt even attempt this. the close minded wont accept what you're doing unless you're black. they will just call you dirty and say you dont wash your hair and your employer wont have it at his business. only way to go, is to make them all neat and clean, which i cant stand, which is why i ended up cutting my hair.