I reached lvl 3 with my fighter charater and about to choose subclass, but on our last quest I found a book with wizard spells. What are my options to be able to use those spells? 

I can't take wizard as a multiclass since I don't have enough INT (10).

Sidenote: our only spellcaster is a druid and no one has enough int to multiclass into a wizard :P

Sell the spellbook and buy some heavy armour with the money.

Take a feat at level that allows you to use a spellbook.

Spellbooks are a pretty uniquely wizard thing. Even Eldritch Knights and Arcane Tricksters don't use spellbooks even though they pick their known spells from the wizard spell list. And with a low intelligence these would not be good subclass choices anyways.

The only way you could make use of the spellbook is if you or someone else picks up the Ritual Caster feat. This would only let you copy and use any ritual spells in the spellbook though.

If there are ritual spells in the book, you could take the feat Ritual Caster (Wizard), and learn them.

I almost have to wonder why your DM gave you a spellbook to start with, if no one in the party is able to use it.  Was it some kind of random loot pulled from a loot table?  If not,  I'd have to wonder if the DM gave the book to you as a potential plot hook of some kind--that is, the important thing isn't the spells in it, but rather who it originally belonged to, or something else.

Honestly, I think just selling the spellbook is the best option if there isn't anything special about it otherwise.  Nobody needs to turn their whole character build upside-down for a spellbook found at level 2.

We are playing the Phandelver adventure and I just picked up the book from the pedestal in the gnomes caves while no one was around. 

Even if you aren't a wizard, a spellbook is a very valuable item. I have a few suggestions for what you could do with it. Of course, you should check with your DM to see how available each of these options is to you before attempting them.

• Sell it! A low-level spellbook isn't exactly a priceless magical treasure, but aspiring wizards and low-level ones trying to learn new spells will both be willing to pay some decent money for your tome. Alternately, a wizard might be willing to do you a large magical favor in exchange for your spellbook.
• Add a wizard to the party! Perhaps one of those spellbook buyers I was talking about is too poor to pay gold for your book, and will instead pay for it with service. If you have a friend who wants to join your D&D group, this is a great opportunity to introduce their character to the campaign, but you could also introduce such a wizard as a TCoE spellcaster sidekick for someone in the group.
• Craft spell scrolls! If your party's druid is proficient with the Arcana skill, the XGtE rules state that they can scribe spell scrolls. Although the rules don't specifically allow it, they might be able to scribe spell scrolls using the spellbook as a reference.

I kind of disagree with the concept of party focused loot. I think loot should more be adventure based With minor adjustments for party balancing.

But oftentimes odd loot allows the party to make unique narratives. Whether it's sold or used as an npc bribe or becomes part of the characters narrative, it becomes interesting.

Now ways to use a spell book.

1. Take a level of wizard (sounds like not an option for op)

2. Take ritual caster wizard (or druid) if any compatible spells.

3. Have someone start warlock and "book of ancient secrets"

3. Save as investment. Proof of bounty. Hope for a trade or bribe. See if dm will reward it's aquisistion later.

4. sell it and be done with it.