How can I get the collection of a custom module in magento 2

You need to inject the model collection factory into your constructor

protected $mymodulemodelFactory;
public function __construct(
    ....
    \[Namespace]\[Module]\Model\ResourceModel\[Entity]\CollectionFactory $mymodulemodelFactory, 
    ...
)
{
    ...
    $this->mymodulemodelFactory = $mymodulemodelFactory;
    ...
}

and you can use in any one of the class methods:

$collection = $this->mymodulemodelFactory->create();

You need to create three files for getting collection from custom module

1) Model, 2) ResourceModel, 3) Collection

Namespace: Prince

Module: Hello

Table: hello

1) Create Hello.php Model file at

app/code/Prince/Helloworld/Model/Hello.php

<?php

namespace Prince\Helloworld\Model;

use Magento\Framework\Model\AbstractModel;

class Hello extends AbstractModel
{
    /**
     * Define resource model
     */
    protected function _construct()
    {
        $this->_init('Prince\Helloworld\Model\ResourceModel\Hello');
    }
}

2) Now create Hello.php ResourceModel at

app/code/Prince/Helloworld/Model/ResourceModel/Hello.php

<?php

namespace Prince\Helloworld\Model\ResourceModel;

use Magento\Framework\Model\ResourceModel\Db\AbstractDb;

class Hello extends AbstractDb
{
    /**
     * Define main table
     */
    protected function _construct()
    {
        $this->_init('hello', 'id'); //hello is table of module
    }
}

3) Now create Collection.php Collection file at

app/code/Prince/Helloworld/Model/ResourceModel/Hello/Collection.php

<?php

namespace Prince\Helloworld\Model\ResourceModel\Hello;

use Magento\Framework\Model\ResourceModel\Db\Collection\AbstractCollection;

class Collection extends AbstractCollection
{
    /**
     * Define model & resource model
     */
    protected function _construct()
    {
        $this->_init(
            'Prince\Helloworld\Model\Hello',
            'Prince\Helloworld\Model\ResourceModel\Hello'
        );
    }
}

Now you can test your collection in Index.php controller app/code/Prince/Helloworld/Controller/Index/Index.php

<?php

namespace Prince\Helloworld\Controller\Index;

use Magento\Framework\App\Action\Action;
use Magento\Framework\App\Action\Context;
use Prince\Helloworld\Model\HelloFactory;

class Index extends Action
{
    protected $_modelHelloFactory;

    public function __construct(
        Context $context, 
        HelloFactory $modelHelloFactory
    ) {
        parent::__construct($context);
        $this->_modelHelloFactory = $modelHelloFactory;
    }

    public function execute()
    {

        $resultPage = $this->_modelHelloFactory->create();
        $collection = $resultPage->getCollection(); //Get Collection of module data
        var_dump($collection->getData());
        exit;

    }
}

If you want to get collection in template using block file

Another four files you need to add nad create

1. Block File

2. Layout File

3. Controller File

4. Templarte File

Create Block file Hello.php

namespace vendorName\ModuleName\Block\Index;

use Magento\Framework\App\RequestInterface;
use vendorName\ModuleName\Model\Hello;

class Hello extends \Magento\Framework\View\Element\Template 
{
    protected $helloModelFactory;
    protected $request;

    public function __construct(
        \Magento\Catalog\Block\Product\Context $context,
        Hello $helloModelFactory,
        RequestInterface $request,
        array $data = []
    ) {
        parent::__construct($context, $data);
        $this->_helloModelFactory = $helloModelFactory;
        $this->_request = $request;
    }

    /**
     * Preparing global layout
     *
     * @return $this
     */
    protected function _prepareLayout() {
        parent::_prepareLayout();
        $this->pageConfig->getTitle()->set(__('Display Collection'));
        return $this;
    }

    public function getHello()
    {
        $collection = $this->_helloModelFactory->getCollection();
        return $collection;
    }
}

Or Create Layout file routName_controllername_index.xml

<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceContainer name="content">
             <block class="vendorName\ModuleName\Block\Event\Index" name="newevent" template="vendorName_ModuleName::Index.phtml" />
        </referenceContainer>
    </body>
</page>

Create Controller File Index.php

<?php
namespace vendorName\ModuleName\Controller\Index;

use Magento\Framework\App\Action;
use Magento\Framework\View\Result\PageFactory;

class Index extends \Magento\Framework\App\Action\Action 
{
    /**
     * @param Action\Context $context
     */

    protected $resultPageFactory;

    public function __construct(
        Action\Context $context,
        PageFactory $resultPageFactory
    ) {
        parent::__construct($context);
        $this->resultPageFactory = $resultPageFactory;
    }

    public function execute() {

        $this->_view->loadLayout();
        $this->_view->renderLayout();
    }
}

Create Template file Index.phtml

<?php

var_dump($block->getgetHello()->getData());

I hope it's work for ever