Can I have a Planeswalker as my Commamder if it doesn't specify that it can be? Obviously {Ob Nixilis of the Black Oath} can be a Commander, but what about {Elspeth} or something?
Also, can I have any legendary card be my Commander? Like {Urborg},{Elbrus}, etc?
Only planeswalkers that have the text stating that they can commanders can be used as commanders. Other than that, only legendary creatures (that start out as legendary creatures) can be commanders.
{Ob Nixilis of the Black Oath} can be
{Ob Nixilis, Unshackled} can be
{Tamiyo, the Moon Sage} cannot be
{Scarmaker} cannot be, since it starts out as a regular creature
{Withengar Unbound} cannot be, as it starts out as an artifact
In other words your commander has to be a legendary creature That starts out a legendary creature. So {Progenitus} can be my commander but {Genju of the Realm} cannot. Neither can {Urborg} or {Urborg Tomb of Yawgmoth}
To be fair, the comprehensive rules are rather vague.
903.3. Each deck has a legendary creature card designated as its commander. This designation is not a characteristic of the object represented by the card; rather, it is an attribute of the card itself. The card retains this designation even when it changes zones.
Example: A commander that's been turned face down (due to Ixidron's effect, for example) is still a commander. A commander that's copying another card (due to Cytoshape's effect, for example) is still a commander. A permanent that's copying a commander (such as a Body Double, for example, copying a commander in a player's graveyard) is not a commander.
903.3a Some planeswalker cards have an ability that states the card can be your commander. This ability modifies the rules for deck construction, and it functions before the game begins. See also rule 112.6m.