سلام، من یک کتاب میخواهم.

In standard Persian orthography it’s usually written with a half‑space (zero‑width non‑joiner) between می and خواهم: می‌خواهم.
You’ll also see میخواهم (no half‑space) in informal typing, but می‌خواهم is the “cleaner” standard form.

A common colloquial pronunciation is:
salâm, man ye ketâb mikhâm.
A more careful/formal pronunciation is closer to:
salâm, man yek ketâb mikhâham.
Notes:

• سلام = salâm (second syllable stressed)
• یک often becomes ye in speech
• می‌خواهم is often shortened to mikhâm in conversation

می‌ is a prefix that typically marks the imperfective/present habitual form in Persian. In many everyday contexts, می‌خواهم is simply the normal way to say I want / I would like.

Yes. یک (yek, meaning a/one) usually comes before the noun:

• یک کتاب = a book / one book
  In speech, یک often becomes یه (ye), but the position stays the same.

Yes. Both are grammatical:

• من یک کتاب می‌خواهم
• من می‌خواهم یک کتاب
  The first one keeps the object (یک کتاب) earlier; the second one puts the verb phrase earlier. In normal conversation, both occur.

You can add را to mark a definite/identified direct object, but it changes the nuance:

• یک کتاب می‌خواهم = I want a book (any book)
• یک کتاب را می‌خواهم = I want that/this one book (more specific, or emphasizing the object)
  Often, when it’s truly indefinite (a book), people omit را.

می‌خواهم is neutral and common, but Persian often uses softer phrasing for politeness. Alternatives:

• یک کتاب لطفاً. = A book, please.
• می‌شه یک کتاب بدین؟ = Could you give me a book?
• ممکنه یک کتاب داشته باشین؟ = Might you have a book?
  Also, adding لطفاً (please) helps.

You don’t need it grammatically, but it’s natural socially. Persian interactions often begin with a greeting before requests. Without it, the sentence can sound abrupt, especially with strangers.

• یک (yek) is the standard written form and also used in careful speech.
• یه (ye) is the very common spoken form (informal pronunciation).
  Meaning-wise, they’re usually the same: a/one.

You can drop it because the verb already shows the person:

• یک کتاب می‌خواهم. = (I) want a book.
  Including من adds emphasis or contrast (like I want a book, not someone else).

Persian doesn’t form the present tense the same way English does. می‌خواهم is a single verb form meaning I want / I would like; it doesn’t need a separate am.

Add نـ to the verb:

• سلام، من یک کتاب نمی‌خواهم.
  Standard spelling: نمی‌خواهم (often typed without half-space as نمیخواهم).