Does anybody know if joining certain guilds/factions affect any other ones you've joined..for example if you join the Dark Brotherhood and the Thieves Guild will there be some kind of conlflict?

No, only the civil war has any effect on other guilds, and that is only the other side of the civil war.

Alright thanks

Actually the dark brotherhood and thieves guild are on friendly terms. But joining one guild than another really has little to no effect.the only factions that can't be joined at the same time is the civil war factions.