Trac will allow you to specify an order for custom fields, but there’s currently no way to change the order of inbuilt ticket fields, either out of the box or through any of the plugins on Trac Hacks.
You can still change the order, but it involves taking a copy of the ticket template source file and modifying it directly. It’s not hard, but might be a little scary for newbies. You’ll also need to be wary in case the original ticket template file is changed by the Trac team, in which case your modified copy will need updating.
Full instructions are available here.