Last night i was playing with my friends as Tyrell and had 2 footmen trying to invade martell's capital, that had 0 units defending it and the +2 bonus of the capital and a single ship supporting the area.
My question is, if the capital has no soldiers do the supporting ship add its bonus and i have to enter a battle to claim that castle or just capture it without figthing?

 That isn't a bonus, that is a garrison. It is a single two strength unit that can't move. Not sure if you can give them orders though.

Unfortunately you can only give support to units and the garrison isn't a unit of any sort. 

Marching into an area with a garrison causes a battle, even if there are no units there.  A defending player can always be supported, even if there are no defending units.

The garrison is not a unit, so it does not count against supply, and it cannot receive an order.  Its only function is to cause the battle to occur and to provide two defensive combat strength.

I concur with dypaca's answer.
I can't find anything in the rules to support a unit requirement for support as suggested by Prince Capsicum.

FYI, check page 26 in the manual.

 I just read page 26 and I still stand by what I said. You can only give another unit support and the rules state that the garrison is not a unit. 

Where do the rules say "You can only give another unit support"?

From the Rulebook:

"A player who controls such an adjacent Support order may now grant (or refuse to grant) his supporting Combat Strength to either the attacker or the defender."

"Support may be given to any adjacent combat, whether a supporting player’s own forces are in the combat, or the combat is between two other players."

That would seem to include Garrisons, no?

rulebook page 26 under GARRISONS

paragraph 3:

If a home area containing a Garrison is attacked, the strength
of the Garrison (the value printed on the token) is added to the
defender’s initial Combat Strength (see page 17). If there are no
units defending a location with a Garrison token, combat still
occurs as normal as if the Garrison was a single unit.

 I bolded and italicized the text that is important.

that should answer the questions

just to clear some other confusion up,

combat only occurs between units. (calculating strengths and playing cards)

Whenever a player marches one or more of his units into an
area containing units from another House, combat ensues.(page 17 of rulebook)

 Hmm that gives me a lot to reflect on. Thanks for the clarification and sorry for the confusion.

jhagen said:

Can you support an embattled area if it has only a power token on it (no units)?

Do you use house cards?

this is discussed in other posts in these forums.

There are 2 conflicting arguments on the matter. Both have validity.

1. a region that is empty cannot be supported. If at any time there are no units to support in the embattled region, combat calculations stop and the opposing army takes control (or keeps control) immediately.

2. a region that was occupied at the start of combat may still be supported even if a card played removed the last unit from the board.

I personally use rule 1. Many others use rule 2. Both are correct when interpreting the rules to support either argument. FFG hasn't clarified this confusion at the moment.

From FFG's FAQ:

"Q: If Mace Tyrell’s text ability destroys the last
remaining defending unit, does the combat still
continue?
A: Yes."

and as discussed above there is nothing in the rules saying that you must have units to be supported.

However I suspect Joeyk1985 was talking about marching into an area which has only a power token (no units or garrison).  In this case there is no battle and the power token is discarded.  It does not even count as an attack, meaning you can do this to multiple areas with one march order.

dypaca said:

Thanks for the clarifications dypaca and jhagen…

And yes,dypaca, i meant exactly that…