Sample Push Topic:

PushTopic pushTopic = new PushTopic();  
pushTopic.Name = 'AccountUpdates';  
pushTopic.Query = 'SELECT Id, Name, Phone FROM Account WHERE BillingCity=\'San Francisco\'';  
pushTopic.ApiVersion = 48.0;  
insert pushTopic; 

Sample Lightning Web Component:

HTML:

<template>  
  
	<lightning-card title="EmpApi Example" icon-name="custom:custom14">  
  
		<div class="slds-m-around_medium">  
  
			<p>Use the buttons below to Subscribe and Unsubscribe!!!</p>  
			<lightning-input label="Channel Name" value={channelName}  
							 onchange={handleChannelName}></lightning-input><br/>  
			<lightning-button variant="success" label="Subscribe" title="Subscribe"  
							  onclick={handleSubscribe} disabled={isSubscribeDisabled}  
							  class="slds-m-left_x-small"></lightning-button>  
			<lightning-button variant="destructive" label="Unsubscribe" title="Unsubscribe"  
							  onclick={handleUnsubscribe} disabled={isUnsubscribeDisabled}  
							  class="slds-m-left_x-small"></lightning-button><br/><br/>  
			<b>Response:</b> <br/>{strResponse}  
  
		</div>  
  
	</lightning-card>  
  
</template> 

JavaScript:

import { LightningElement, track } from 'lwc';  
import { subscribe, unsubscribe, onError, setDebugFlag, isEmpEnabled } from 'lightning/empApi';  
  
export default class PushTopicLWC extends LightningElement {  
  
	@track channelName = '/topic/AccountUpdates';  
	@track isSubscribeDisabled = false;  
	@track isUnsubscribeDisabled = !this.isSubscribeDisabled;  
	@track strResponse = '';  
  
	subscription = {};  
  
	// Tracks changes to channelName text field  
	handleChannelName(event) {  
		this.channelName = event.target.value;  
	}  
  
	// Handles subscribe button click  
	handleSubscribe() {  
		// Callback invoked whenever a new event message is received  
		const messageCallback = ( response ) => {  
			console.log( 'New message received : ', JSON.stringify( response ) );  
			// Response contains the payload of the new message received  
			this.strResponse = JSON.stringify( response );  
			console.log( this.strResponse );  
		};  
  
		// Invoke subscribe method of empApi. Pass reference to messageCallback  
		subscribe( this.channelName, -1, messageCallback ).then(response => {  
			// Response contains the subscription information on successful subscribe call  
			console.log( 'Successfully subscribed to : ', JSON.stringify( response.channel ) );  
			this.subscription = response;  
			this.toggleSubscribeButton( true );  
		});  
	}  
  
	// Handles unsubscribe button click  
	handleUnsubscribe() {  
		this.toggleSubscribeButton( false );  
  
		// Invoke unsubscribe method of empApi  
		unsubscribe(this.subscription, response => {  
			console.log( 'unsubscribe() response: ', JSON.stringify( response ) );  
			// Response is true for successful unsubscribe  
		});  
	}  
  
	toggleSubscribeButton( enableSubscribe ) {  
  
		this.isSubscribeDisabled = enableSubscribe;  
		this.isUnsubscribeDisabled = !enableSubscribe;  
  
	}  
  
	registerErrorListener() {  
  
		// Invoke onError empApi method  
		onError(error => {  
			console.log( 'Received error from server: ', JSON.stringify( error ) );  
			// Error contains the server-side error  
		});  
  
	}  
  
} 

JS-Meta.xml:

<?xml version="1.0" encoding="UTF-8"?>  
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">  
	<apiVersion>48.0</apiVersion>  
	<isExposed>true</isExposed>        
	<targets>      
		<target>lightning__Tab</target>      
	</targets>  
</LightningComponentBundle> 

Output: