Question
how do i keep weeds out of my grass without hurting my roses and trees.

Answer
Most liquid herbicides will not harm the tree or the roses assuming that the spray is not applied to the foliage of the trees or roses. You will need to determine what type of weed you have in the grass to effectively control them. DO NOT use a granular herbicide or a product called Weed and Feed. These will cause damage to trees.  

Here is a web link to information on weeds in lawns. http://www.extension.umn.edu/distribution/horticulture/DG1137.html