Hi, i'm begginner with inkscape and want to know how can i remove the outline (stroke) from rectangle's top ?????

Welcome to InkscapeForum!

If the rectangle is not going to have any fill color, you can do it this way:
-- Path menu > Object to path
-- Select the segment you want to remove with the Node tool.
-- Click "Delete segment between 2 non-endpoint nodes", which is the 6th button from the left on the tool control bar.

If it IS going to have a fill, and it's a true rectangle, you can still do it that way and the fill won't be affected. But in general, if you want an object to have a partial border, the best way to do it is to duplicate the object. Remove the stroke on the original, and remove the fill on the duplicate. Then edit the duplicate path to however you want it.

Happy Inkscape-ing!

Yeah man, i got it!!! Thanks!!!

You can also fake it by using a gradient for the stroke. Set one end stop to transparent and the other to the colour of your stroke, then you can position them very close together to make one end of the rectangle transparent, with a very small gradient blend between that and the rest of the stroke - giving it a "soft" edge.

By adding extra stops to the gradient it's possible to create a "hard" transition from transparent to coloured as well.

This approach can be useful if you need the rectangle to remain as a rectangle for whatever reason, rather than becoming a path - but for most situations I'd go with Brynn's suggestion.