<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <tab id="mymodule" translate="label" sortOrder="100">
            <label>My Module</label>
        </tab>
        <section id="mymodule" translate="label" type="text" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="1">
            <label>My Module</label>
            <tab>myconfig</tab>
            <resource>Vendor_Module::system_config</resource>
            <group id="general" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>General Configuration</label>
                <field id="enable" translate="label" type="select" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Enable Module</label>
                    <source_model>MagentoConfigModelConfigSourceEnabledisable</source_model>
                </field>
            </group>
        </section>
    </system>
</config>

class Myclass
{
    public function __construct(
        MagentoFrameworkAppConfigScopeConfigInterface $scopeConfig
    ){
        $this->scopeConfig = $scopeConfig;
    }

    public function isModuleEnabled($website = null)
    {
        return $this->scopeConfig->isSetFlag(
            'mymodule/general/enable',
            MagentoStoreModelScopeInterface::SCOPE_WEBSITE,
            $website
        );
    }

    public function execute()
    {
        if ($this->isModuleEnabled()) {
            // Do stuff here if module is enabled.
        }
    }
}