An example showing the default boilerplate code for a new client web part (from preview tooling).

CobLatestNewsWebPart.ts

import {
 IWebPartData,
  IWebPartContext,
  IClientSideWebPart,
  BaseClientSideWebPart,
  DisplayMode,
  IPropertyPanePage,
  IPropertyPaneFieldType
} from '@ms/sp-client-platform';

import './CobLatestnewsWp.css';
import Strings from './CobLatestnewsWp.strings';

export interface ICobLatestnewsWpWebPartProps {
  description: string
}

export class CobLatestnewsWpWebPart extends BaseClientSideWebPart<ICobLatestnewsWpWebPartProps> {

  public constructor(context: IWebPartContext, loadedModules: { [moduleName: string]: any }){
      super(context, loadedModules);
  }

  public render(mode: DisplayMode, data?: IWebPartData) {
    this.domElement.innerHTML = `
            <div class="CobLatestnewsWp">
                <div class="Container">
                    <div class="ms-Grid-row ms-bgColor-themeDark ms-fontColor-white intro">
                        <div class="ms-Grid-col ms-u-lg10 ms-u-xl8 ms-u-xlPush2 ms-u-lgPush1">
                            <span class="ms-font-xl ms-fontColor-white">Welcome to SharePoint!</span>
                            <p class="ms-font-l ms-fontColor-white">Customize SharePoint experiences using Web Parts.</p>
                            <p class="ms-font-l ms-fontColor-white">${this.properties.description}</p>
                            <a href="https://github.com/OfficeDev/SharePointUXFramework/wiki" class="ms-Button"><span class="ms-Button-label">Learn more</span></a>
                        </div>
                    </div>
                </div>
            </div>`;
  }

  protected get propertyPaneSettings(): IPropertyPanePage[] {
    return [{
      header: {
        title: Strings.PropertyPaneHeader
      },
      groups: [
        {
          groupName: Strings.BasicGroupName,
          groupFields: [
            {
              type: IPropertyPaneFieldType.TextBox,
              targetProperty: 'description',
              properties: {
                label: Strings.DescriptionFieldLabel
              }
            }]
        }]
    }];
  }
}