The easiest way to gain healthy muscle weight is through weight training and proper diet. By eating high protein (muscle building foods) combined with lifting weights you can add muscle mass to your body.
If a person just wants to tone up with weight lifting the process involves lots of repetition of light weights. To gain muscle mass the process involves less repetition, but greater weight. Since you want to gain weight and muscle, I would suggest using weights that are within your ability, but not too heavy (you don't want to hurt yourself), and do less repetition. Also, muscle weighs more than fat, so by doing this type of training you will gain good weight faster than you would by putting on fat weight.
Never do any heavy lifting of free weights without a "Spotter", someone there to assist you with the weight, just in case you have a problem. I had a good friend who was doing "French Curls" with free weights, only to have the clamp come loose allowing the weights to slide off, and hit him in the head. He was knocked out, and needed several stitches for the gash on his forehead, and had he been alone at the time this accident might have been much worst. So, as with all lifting be careful with how you lift. I wish you good fortune in your endeavor.