Unlike the standard ば form of hypothetical phrases, ねば makes use of the classical Japanese auxiliary verb ず in its ね form. However, the meaning is the same as the modern なければ ‘if not (A)’. ねば connects to verbs in the same way as ない, and will then be followed by the negative form of なる ‘to become’, ならない.
Like
なければならない,
ねばならない will translate to ‘must do (A)’, or ‘have to do (A)’, and is simply a double negative that literally means ‘if not (A), it won’t be (B)’. This is a formal structure, and will mostly be found in literature.
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今月（きょう）の13日（にち）までに電気代（でんきだい）を払（はら）わねばならない。
I have to pay the electric bill by the 13th of this month.
あの人は一応（いちおう）私（わたし）の先輩（せんぱい）なので何（なに）を言（い）われても、従（したが）わねばならない。
That person is my senpai, more or less, so no matter what he says, I must obey him.
As ね is a form of the auxiliary verb ず, the base of する will change to せねば, in the same way it changes to せず in other grammar patterns.
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お客（きゃく）さんが来（く）る前（まえ）に掃除（そうじ）をせねばならない。
I must clean before the guests arrive.
日本語（にほんご）を今（いま）より話（はな）せる様（よう）になりたいのならば、もっと努力（どりょく）をせねばならない。
If you want to be able to speak Japanese better than you do now, you have to work harder.
Caution - Sometimes ならない will be replaced with ならぬ. This has exactly the same meaning and just makes use of the ぬ attributive form of the same auxiliary verb, ず.
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社長（しゃちょう）には僕（ぼく）に任（まか）せてくださいと言（い）ったので、何（なに）があっても成功（せいこう）せねばならぬ。
I told my boss to leave it to me, so no matter what happens, I must succeed.
While not changing the meaning at all, the addition of ぬ will make the overall expression sound even more old-fashioned.
Fun-fact - As with other ‘must’ double-negative structures, the second part of the ねばならない may sometimes be omitted, resulting in ねば or せねば appearing by itself. This is similar to the modern ないと appearing without いけない following it.
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もうこんな時間（じかん）ですか、そろそろ帰（かえ）らねば。
It’s already so late! I must go home soon.
あなたたちは兄弟（きょうだい）でしょう？もっと仲良（なかよ）くせねば。
You guys are siblings, aren’t you? You guys have to be nice to each other.