Siempre ayudo a quien me necesita.

Because ayudo is the first-person singular present tense of ayudar:

• yo ayudo = I help
• él/ella/usted ayuda = he/she/you help(s)

Since the sentence means I always help..., the verb has to be ayudo.

That a is the personal a, which is used before a human direct object in Spanish.

Here, the speaker is helping a person:

• Siempre ayudo a quien me necesita.

So a quien means something like whoever or the person who.

You often see this a after verbs when the object is a person:

• Veo a mi hermano.
• Ayudo a mi amiga.
• Escucho a quien habla.

Even though English does not use a special marker here, Spanish usually does.

Here quien means whoever, anyone who, or the person who.

So:

• Siempre ayudo a quien me necesita
  = I always help whoever needs me

It is a relative pronoun. It does not ask a question here; it connects the main idea (I always help) with the description of the person (the one who needs me).

Because quien here is a relative pronoun, not a question word.

• quien = who / whoever / the one who in a relative clause
• quién = who? in direct or indirect questions

Compare:

• Ayudo a quien me necesita. = I help whoever needs me.
• ¿Quién me necesita? = Who needs me?

A useful rule: question words usually take the accent when they are used in questions or exclamations.

Because me means me as the direct object of necesitar.

• me necesita = needs me

In the clause quien me necesita, the subject is quien:

• quien = whoever / the person who
• me = me
• necesita = needs

So literally, it is:

• whoever needs me

Spanish normally uses object pronouns like me, te, lo/la, nos before the conjugated verb:

• me necesita
• te necesita
• nos necesita

Because the subject of that verb is quien.

Even though quien can mean whoever or the person who, grammatically it is treated here as singular, so the verb is singular too:

• quien necesita = whoever needs

That is why you get:

• a quien me necesita

not:

• a quien me necesitan

If you used quienes instead, then a plural verb would be possible:

• Ayudo a quienes me necesitan.

Yes. That is also correct.

• a quien me necesita = whoever needs me / the person who needs me
• a quienes me necesitan = those who need me / the people who need me

The singular quien is very common for a general statement. It can sound broad and natural, almost like whoever in English.

The plural version makes the idea feel more explicitly plural:

• Siempre ayudo a quienes me necesitan.
  = I always help those who need me.

Both are good; the original is perfectly natural.

Because Spanish usually omits subject pronouns when the verb already shows who the subject is.

• ayudo already means I help
• so yo is not necessary

You could say:

• Yo siempre ayudo a quien me necesita.

But adding yo gives extra emphasis, as if you were contrasting yourself with someone else:

• Yo siempre ayudo..., pero él no.

Without that contrast or emphasis, omitting yo is more natural.

Putting siempre first is very natural and emphasizes the idea of always right away.

• Siempre ayudo a quien me necesita.

This word order is standard and smooth in Spanish.

You can move adverbs around sometimes, but not every position sounds equally natural. For example:

• Siempre ayudo a quien me necesita. ← most natural
• Yo siempre ayudo a quien me necesita. ← also natural
• Ayudo siempre a quien me necesita. ← possible, but less neutral

So the original sentence uses the most common and natural placement.

Because the sentence expresses a habit, general truth, or regular behavior.

• Siempre ayudo... = I always help...

Spanish often uses the present tense for things you do habitually:

• Siempre desayuno temprano.
• Nunca fumo.
• Ayudo a quien me necesita.

It does not mean only right now. It can also mean in general.

Not in this exact sentence, because it states a general fact/habit:

• Siempre ayudo a quien me necesita.

Here, me necesita is in the indicative because the speaker is talking about people who actually need them, in a real and general sense.

But in a different context, you might see the subjunctive:

• Ayudaré a quien me necesite.
  = I will help whoever may need me.

Here necesite sounds more open or future-oriented. So:

• me necesita = whoever needs me / the one who needs me
• me necesite = whoever might need me

Both are possible in Spanish, but they express slightly different ideas.