I have a 1st-level sorcerer in my game that has a rabbit familiar. Familiars are magical beasts, so you can't use "speak with animals" or anything that specifically affects animals. As a result, she can't actually communicate with her familiar at all.
Other than to be an intelligent (INT 6) creature, the familiar seems to largely just be a liability since it's not like she can give it instructions. It sits in her backpack with its head poking out all the time but otherwise doesn't do anything.
So… what's the point of having a familiar at 1st level anyway?
Admittedly, she could get it a pearl of speech (MIC p118; 600 gp; Item Level 3rd) to grant it the ability to speak and understand a specific language, but that's quite a ways off for a 1st-level character. It seems to me that the only immediately useful abilities of a 1st-level familiar are the passive skill benefits (Alertness, and the animal-specific benefit), the ability to make it slightly less of a liability by sharing spells with it, and the possibility of leaving it to guard something so it can empathically inform you when something scary is nearby. However, that last one is useless to an adventuring sorcerer that's going to regularly be more than a mile away.
Best Answer
Not sharing a language doesn't mean you can't communicate. Familiars are smart (6 intelligence at level 1, and rising fast) and have empathic links to their associated spellcasters. Which is to say, you can demonstrate concepts to your familiar by example, even if you can't do it through speech; And you can readily communicate an emotion associated with each concept you communicate. It's less convenient than speech, admittedly; It takes a lot of effort to explain a concept without words, and it's unlikely your familiar shares enough of your cultural background that you'll be able to do it without starting from first principles... But it's enough to teach your familiar to act as a lookout for approaching humanoids, or to keep an eye on the soup and let you know if it starts to boil over.
Even without specific instruction, the empathic link can be surprisingly useful. In a campaign I was running, the Wizard opted not to take his familiar (an owl) into an undead-infested dungeon, and instead left it to wait in the blasted, deserted wasteland outside - and so was able to guess something was up outside when he later felt surprise and fear without there being anything fearsome or surprising in sight.