Jem kolację w kuchni.

Jeść is the infinitive (to eat). Jem is the 1st person singular present tense form (I eat / I am eating).
Conjugation (present tense) of jeść:

• (ja) jem – I eat
• (ty) jesz – you eat
• (on/ona/ono) je – he/she/it eats
• (my) jemy – we eat
• (wy) jecie – you (pl.) eat
• (oni/one) jedzą – they eat

Both. Polish present tense commonly covers both habitual and “right now” meanings, and context decides:

• With no extra context, Jem kolację is often understood as I’m eating dinner (now).
• For habitual meaning (I eat dinner [regularly]), you might also hear Jadam kolację (more clearly habitual).

Because it’s the direct object of jem (I eat [what?]), so it takes the accusative case.
For feminine nouns ending in -a, the accusative singular usually changes:

• kolacja (nominative: “dinner” as a subject)
• kolację (accusative: “dinner” as an object)

w kuchni uses the locative case because w meaning in/inside (static location) governs the locative.

• kuchnia (dictionary form, nominative)
• w kuchni (locative: in the kitchen)

w kuchnię is used with motion toward/into something (accusative after w), e.g.:

• Idę w kuchnię. – I’m going into the kitchen.
  With being/doing something in a place (no movement), it’s w kuchni:
• Jem kolację w kuchni. – I’m eating dinner in the kitchen.

Yes. Polish word order is flexible and changes emphasis:

• Jem kolację w kuchni. – neutral
• W kuchni jem kolację. – emphasis on where (in the kitchen, not elsewhere)
• Kolację jem w kuchni. – emphasis on what (dinner, not something else)

Usually no. The verb ending in jem already shows it’s I. You add ja mainly for emphasis or contrast:

• Jem kolację w kuchni. – normal
• Ja jem kolację w kuchni, a on w salonie. – I eat in the kitchen, and he in the living room.

Approximate pronunciation:

• Jem ≈ yem (like yes with m; j is like English y)
• kolację ≈ ko-LA-tsyeh (the ę at the end is often close to an eh sound; it’s not a clear en in this position)
• w kuchni ≈ f KOOH-nee (the w often sounds like f before a voiceless consonant like k)
  Also: ch is a guttural sound like German Bach (not English ch).

In Polish, kolacja is typically the evening meal/supper. Depending on the household/culture, English dinner may correspond to:

• obiad = often the main hot meal (frequently earlier than evening, e.g., afternoon)
• kolacja = evening meal
  So kolacja can match English dinner in some contexts, but often it’s closer to supper.

Yes—grammatically it’s the same pattern:

• Jem obiad w kuchni. – I’m eating lunch/dinner (the main meal) in the kitchen.
  Here obiad is masculine, and its accusative singular is the same as nominative (obiad), so it doesn’t change form.

Use nie before the verb:

• Nie jem kolacji w kuchni. – I’m not eating dinner in the kitchen.
  Notice the object often changes to genitive in negation:
• kolację (accusative, affirmative)
• kolacji (genitive, negative)

Common options:

• Jesz kolację w kuchni? – Are you eating dinner in the kitchen? (to one person)
• Czy jesz kolację w kuchni? – Same, slightly more explicit (with czy)
• Gdzie jesz kolację? – Where are you eating dinner?
• Co jesz w kuchni? – What are you eating in the kitchen?