See Playing as a vassal for more details. However, you are still vulnerable to claims from other vassals within the realm.
Playing as a vassal of the head of a large realm allows you to have a liege protect you from outside threats. For beginners, the safest starts as independent kings are King Boleslaw 'the Bold' of Poland and King Svend of Danmark. You have to be prepared for malcontent vassals and the eventuality of a fairly large war with a neighboring realm. Though you probably will not be wiped out immediately, internal strife caused by power-seeking vassals can quickly leave a large realm vulnerable. The head of a big kingdom or empire starts off with a lot of responsibility. Starting as King Murchad of Mumu in Ireland in 1066 is generally considered the easiest way to learn the basics of conquest, as all of Ireland's other realms are smaller and the nearby kingdoms will leave it alone for a long time. Consider the following when choosing your first character: There are many options for playable characters and it can be overwhelming trying to get started.
When starting a new game, click Custom Game Setup to choose any date and character. In CKII, you play as a ruler rather than a country.