I have a master-detail view. Which is the best practice to add new items to the list? Now I think when user clicks the new button a new tab item can appear with close button, so the user can navigate back to the list if it is needed. Do you know any better solution? Editing of items can be done in the detail view.
If the details that a person needs to enter for a new item can fit into a row, then I'd also recommend inserting a new row at the top. If there's more detail than can be specified in a row, or if the tools needed to enter the new item are too much for a row, use a dialog.
I don't think a dialog would be distracting in this case since it's in direct response to a person's action of asking for a new item
Judging from your sketch, new items are supposed to appear inside the List tab, while a new tab would mean a new list of items, not a new single item.
If you prefer to edit new items in the details pane, then clicking the New button (where is it btw?) should add a new blank line in the Master pane, make it selected, and set your focus in the first field of the Details pane. However, if it's a smart grid (e.g. that supports sorting), typically I would open a new dialog for the new item, and then place it in the appropriate place in the grid. If the grid is simple, then opening a new line at the top/bottom should be fine.
I didn't get the idea about new tabs. Usually all popups and dialogs are interrupting the users' train of thought and thus to be avoided if possible.
I would keep the first and/or last row of the table always visible and reserved for inserting a new row item (kind of edit-in-place).
When the new row is ready and inserted among the other data it's a must to highlight its new position with a color fade or some appropriate animation.