Hello, im really new in blender so this might be a really newbie question. I made a character, added the bones and joined them with the mesh but when i move a leg the other one tends to follow it. please help.
here are some pictures for a better understanding thanks.
This is a problem of weight, as in “Weight Painting”. The weight corresponding to the bones of one legs goes beyond what it should and includes vertices from the opposite leg.
Easy fix: Edit your mesh and go to the Object Data tab (the blue triangle), you’ll find the Vertex Groups panel.
Step 1: Select all vertices. Select one vertex group matching a leg bone. (The name of the groups match the bones.)
Step 2: Click “Deselect”. You should see vertices deselected on the wrong leg.
Step 3: Select those vertices which shouldn’t be influenced by the bone. Click “Remove”.
Go to Step 1 until you have done all the bones of the legs.
Note that it’s normal for the weight of a bone to extend beyond its ends as long as it’s still on the same limb and/or on the same side of the body. Just don’t remove too much! 
If you have weights from one side of the body crossing the center line to influence the other, that’s almost always due to unapplied scales and/or rotations. Make sure before parenting with automatic weights that you apply these. Ctrl-A in object mode. Should save you a bit of manual tinkering, though Kaluura’s method is correct if manual tinkering is still necessary.