I have created the SharePoint list.I need to show my custom form instead of showing the existing add new form.how to create the custom new form in SharePoint list for add new item ?
For Example :
As @Danny '365CSI' Engelman stated SharePoint designer can/should be part of the solution, but doesn't have to be depending on what version of SharePoint you're using and how your pages come up.
Probably the best way is to use SharePoint designer to add a new form to your list, setting it as the default. You don't say how you developed your custom form, but I'm making the assumption you mean client-side code. If that's true, then edit the page, delete the existing web part "form" and insert a CEWP/SEWP on the page and link to the code for your custom code.
You can also edit the existing form page and do the same process. If your forms are showing up in a dialog box you'll need to turn that off temporarily under advanced list settings, or get the URL for the form (right mouse click over the form body and get properties or get it from SharePoint Designer) and then paste that URL into the browser removing the &IsDlg=1