Necesito planificar mis gastos del mes.

In Spanish, the subject pronoun is often omitted because the verb ending already shows who is doing the action.

• Necesito = I need
• The -o ending tells you it is yo

So Necesito planificar mis gastos del mes is perfectly natural.
You could say Yo necesito..., but that usually adds emphasis, such as I need to... rather than someone else.

Necesito is the first-person singular present tense of necesitar.

• necesitar = to need
• necesito = I need

It follows a regular -ar verb pattern:

• yo necesito
• tú necesitas
• él/ella necesita
• nosotros necesitamos
• vosotros necesitáis
• ellos necesitan

After a conjugated verb like necesito, Spanish often uses an infinitive for the next action.

So:

• Necesito planificar = I need to plan

This is similar to English, where need is followed by to + verb.
In Spanish, you usually do not add a separate word for to here:

• Necesito comer = I need to eat
• Necesito estudiar = I need to study
• Necesito planificar = I need to plan

Yes. Both planificar and planear can mean to plan.

However, planificar often sounds a bit more like to plan carefully / to organize in advance, while planear can be a more general to plan.

In the context of budgets, expenses, or schedules, planificar sounds very natural.

So all of these can work, depending on nuance:

• Necesito planificar mis gastos del mes
• Necesito planear mis gastos del mes

Gastos means expenses or spending.

It refers to money you spend, for example:

• rent
• food
• transport
• bills
• entertainment

So mis gastos = my expenses

This is different from some related words:

• gasto = expense / expenditure
• gastos = expenses
• cuenta = bill/check/account, depending on context
• factura = invoice or bill
• coste / costo = cost

Because gastos is plural, the possessive must also be plural.

• mi gasto = my expense
• mis gastos = my expenses

Spanish possessives agree with the noun in number:

• mi / mis
• tu / tus
• su / sus

So since gastos is plural, mis is required.

Del is a contraction of de + el.

• de = of
• el = the
• del = of the

So:

• del mes = of the month

In natural English, this sentence is usually understood as something like:

• my expenses for the month
• my monthly expenses
• my expenses this month
  depending on context

Spanish often uses de where English might use for.

In Spanish, de + el almost always contracts to del.

So:

• de el mes ❌
• del mes ✅

This is a standard grammar rule.

Another common contraction is:

• a + el = al

For example:

• Voy al banco = I’m going to the bank

One important exception: there is no contraction if El is part of a proper name.

For example:

• de El Escorial ✅

Not necessarily. Literally, del mes means of the month. In context, it often means for the month or this month’s expenses.

If you specifically want to say this month, Spanish would usually say:

• este mes = this month

So:

• mis gastos del mes = my expenses for the month / this month’s expenses
• mis gastos de este mes = my expenses for this month

The original sentence is natural and common, especially when talking generally about budgeting.

Yes. That would also be natural, but the meaning changes slightly.

• Necesito planificar... = I need to plan...
• Tengo que planificar... = I have to plan...

Necesito suggests need or necessity.
Tengo que suggests have to or obligation.

Both are common, but necesito may sound a little more personal or practical in this context.

It is plural because people usually talk about several different expenses, not just one.

• mi gasto del mes = my expense for the month
  this sounds like one single expense
• mis gastos del mes = my expenses for the month
  this is the normal idea when talking about budgeting

So the plural is the natural choice here.

Yes, it is natural and correct in Spain Spanish.

A speaker in Spain might also say similar things like:

• Necesito organizar mis gastos del mes
• Necesito controlar mis gastos del mes
• Necesito planificar los gastos del mes

But Necesito planificar mis gastos del mes is completely normal and clear.

You could say:

• Necesito planificar mi presupuesto del mes = I need to plan my budget for the month
• Necesito organizar mis gastos del mes = I need to organize my monthly expenses
• Necesito controlar mis gastos del mes = I need to keep track of my monthly expenses

The original sentence focuses on the expenses themselves.
If you use presupuesto, you shift the focus more toward the budget as a whole.