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I’m kind of new to the game and I want to make a wizard, but I can’t understand the part of the hero’s handbook (pg.29) where it says:
“Pick a number of spells from this page equal to 3 + INT”

I have 17 + 3 INT, so what do I do?

You know 6 spells as a first level wizard

On the Beginner Box's Hero Handbook about 1st level wizard spells (page 28), it says:

Pick a number of spells from this page equal to 3 + INT. These are the
spells in your spellbook. Write these spell names in the Wizard’s Spellbook area of Section K on your character sheet.

But the key to understand this bolded formula (3+INT) actually lies earlier in the book, on page 13, about Ability Scores:

Use your ability scores and the table below to figure what your Ability Modifiers (also called Ability Mods) are. When you see an abbreviation like STR , DEX, CON , INT , WIS, or CHA , that means your Ability Mod (–5 to +5), not your Ability Score (1–20).

So, whenever you see the ability's name abbreviated and full caps, it means your Ability Modifier, not your ability score.

Your intelligence is 17, that means your intelligence's ability modifier (INT) is +3. So you know 6 spells as a first level wizard.

Here is the full text from the Hero's Handbook (Note - As pointed out in the comments, the core rules have a different standard than what is in the Beginners Box set):

Pick a number of spells from this page equal to 3 + INT. These are the spells in your spellbook. Write these spell names in the Wizard’s Spellbook area of Section K on your character sheet. You can prepare one 1st-level wizard spell per day from the list of spells in your spellbook. If your Intelligence ability score is 13 or higher, you can prepare an extra 1st-level wizard spell each day! Write this number of 1st-level spells per day in Section K. If a spell says you can cast it on an ally, you can instead cast it on yourself—you don’t have to cast it on someone else.

So what that is saying, that with your 17 intelligence, you have a +3 modifier, so you can copy up to 6 spells into your spellbook. These are the spells that you know. However, the limitation is on how many you can prepare per day, or get ready to actually use. Since you are first level, you would normally only be able to prepare one spell per day, but since you have a high intelligence you can prepare 2 spells for use.

You also get 4 different cantrips, or zero level spells. These are Detect Magic, Ray of Frost, Mage Hand and Read Magic (As shown on page 28). So your base total at 1st level with your intelligence will be the 4 cantrips, and 6 spells of your choice, of which you can prepare 2 for use each day. Cantrips do not need preparation and can be used over and over.

Think of it like a buffet menu, there are 6 options, but you can only put two things on your plate at a time. Next time you have a rest, you can prepare new spells, either the same or different. They just have to be inscribed in your spellbook, however.