You're not the head of a civilization, just a small outpost, why would you have the ability to declare war?
That aside, you're in luck. Goblins are naturally hostile to dwarves because they snatch children (not sure if that makes sense, but it's true). So they'll attack you when they feel like it, declaring war in the process automatically.
Unless you've modified goblin settings, they won't think about attacking you (besides child-theft and maybe the odd ambush, although ambushing seems to be broken) until your population reaches 80.
Even then, depending on how many resources they're throwing at the Elves they might be too worn down to attack you.
The next version is out soon. In that you still won't be able to declare war, but you will be able to provoke goblins (or anyone else, even other dwarf civs) to declaring war on you by harassing them with squads and stealing their stuff.
Again, that's a civ level decision, their civ will declare war on your civ. This should mean they'll attack you, but they might be busy attacking all your civ's other sites instead.
What you can do though is build lots of artifacts and a tavern. Spies will make reports on your artifacts and leaders hearing about your stuff may well come and 'ask' you for it. Theoretically this means their armies will come straight to you, ignoring all those elves and other rich sites in between you and them and the glory days of plentiful sieges will be with us again. Maybe.