You gain the abilities on top of your abilities you had for your first class. They simply build on one another. A good class to multiclass with Rogue is Fighter. Go even with rogue and fighter till fourth, the focus on rogue. You can get the extra feats to set yourself up as a decent dexterity based frontliner with the ability to handle most traps and locks without the use of thieves' tools. The two best races for that combination are human and lightfoot halfling. Human for that extra feat at first level and the extra skill points. Lightfoot halfling for the dexterity bonus, the bonus to saves, and the bonus for attack and defense due to size.