Estoy mareado y necesito sentarme en el sofá.

Because mareado describes a temporary physical state, so Spanish uses estar, not ser.

• estoy mareado = I feel dizzy / I am dizzy
• soy is used for identity, origin, permanent traits, etc.

So soy mareado would sound wrong here.

In this sentence, mareado means something like:

• dizzy
• light-headed
• sometimes nauseous
• sometimes motion-sick

In Spain, mareado is a very common word for feeling physically unwell in that way. The exact meaning depends on context.

Because it agrees with a masculine singular speaker.

If the speaker is female, you would say:

• Estoy mareada y necesito sentarme en el sofá.

Other forms:

• mareado = masculine singular
• mareada = feminine singular
• mareados = masculine/mixed plural
• mareadas = feminine plural

Usually no. Spanish often leaves out subject pronouns because the verb ending already shows who is doing the action.

So these are natural:

• Estoy mareado
• Necesito sentarme

Adding yo is possible, but it usually adds emphasis or contrast:

• Yo necesito sentarme, pero él no.

Because the verb here is sentarse, which means to sit down.

• sentarse = to sit down / to sit oneself down
• sentar = to seat someone, to place someone somewhere, or sometimes to suit someone

Examples:

• Necesito sentarme. = I need to sit down.
• Voy a sentar al niño en el sofá. = I’m going to seat the child on the sofa.

So in your sentence, sentarme is the correct form.

It is the infinitive sentar plus the reflexive pronoun me attached to the end:

• sentar + me = sentarme

After a verb like necesito, Spanish often puts the second verb in the infinitive:

• Necesito sentarme. = I need to sit down.

This is very common in Spanish:

• quiero irme
• puedo levantarme
• voy a acostarme

For this sentence, the natural choice is necesito sentarme.

With some verb combinations, Spanish allows the pronoun either before the first verb or attached to the infinitive. But with necesitar, speakers normally prefer:

• Necesito sentarme

Me necesito sentar sounds odd or unnatural to many speakers here, so it is better to avoid it.

In Spanish, with furniture like chairs, sofas, and beds, sentarse en is the normal choice.

• sentarse en el sofá
• sentarse en una silla
• sentarse en la cama

Sobre usually means on top of in a more literal way, often emphasizing physical position on a surface. For a normal sofa situation, en el sofá is what you want.

Because grammatical gender in Spanish is not determined only by the last letter. Sofá is a masculine noun, so it takes el:

• el sofá
• un sofá

This is something you simply learn with the noun. Not all words ending in -a are feminine, and not all words ending in -o are masculine.

Yes, if you mean a specific sofa: the sofa.

• en el sofá = on the sofa
• en un sofá = on a sofa

If you are talking about the sofa in the room, or the sofa everyone knows about, el is the natural choice.

It is pronounced roughly so-FA, with the stress on the last syllable.

The written accent mark tells you where the stress goes:

• sofá

Without the accent, Spanish pronunciation rules would suggest stress elsewhere, so the accent is important.

It can be used for both.

For example, in Spain:

• Estoy mareado can mean I feel dizzy
• Me he mareado en el coche can mean I got carsick

So it is a flexible word for that type of physical discomfort.

In this context, to sit down is the better match.

• sentarse often focuses on the action of moving into a seated position
• English sit can sometimes mean simply be seated

So necesito sentarme is most naturally I need to sit down.

Yes, it sounds natural and completely normal in Spain.

A Spanish speaker in Spain could easily say:

• Estoy mareado y necesito sentarme en el sofá.

It is clear, everyday Spanish, and the vocabulary sounds standard for Spain.