Can i still join The Dark Brotherhood?
I found the boy's home before I actually found the quest instead of doing the mission.
will that still count? will I still be able to join The Dark Brotherhood or will I have to start over again and do the whole Quest step by step
While the answer provided by Yasskier is correct in itself, it doesn't focus on your question; can I continue the dark brotherhood quest even though I have found the home of Aventus Aretino without having been given a quest.
The answer is; yes.
After all, the quest to join the Dark Brotherhood is Innocence Lost which only begins when you speak to Aventus - it cannot begin before this point. So, just go on into the house and speak to him.
The "mission" you may be referring to is the miscellaneous objective "Talk to Aventus Aretino" which can be given to you by asking an innkeeper for rumours. This isn't required to begin the main Dark Brotherhood quest, it's just a rumour that points you towards a potential quest. There's a lot of quests that have a rumour as a "pointer", but you can always start them yourself without ever hearing about the rumour; this is the situation you find yourself in.
Rumours usually come from innkeepers or guards and will result in a miscellaneous objective and a quest marker, but other than that can be ignored or followed at your leisure. Other examples include:
- The Black Star - innkeepers will direct you to the Shrine of Azure, or you can find it yourself and start the quest
- Dawnguard (Quest) - guards will point you towards to newly inhabited Fort Dawnguard, or find it yourself
- Surgery - inkeepers and guards will mention the Face Sculptor in Riften, but you can find her yourself
There are quite a few others too.
The condition of joining (or destroying) the Dark Brotherhood depends on the completing the "Innocence Lost" quest: You need to talk Aventus Aretino in Windhelm and kill Grelod the Kind in the Honorhal Orphanage. After killing Grelod, go to sleep - you will be kidnapped by Astrid and then you have a choice: either kill any of the prisoners if you want to join the brotherhood or kill Astrid if you want to destroy it.
Visiting the Sanctuary before that shouldn't influence the outcome.