Building muscle?

Start lifting NOW!! =D

Lifting will help you retain the muscle you have. You may actually be able to build some muscle while eating at deficit if you have a lot of weight to lose and have your macros on point.

Regardless if you build new muscle or not, you can make amazing strength gains in the first few months. 'Starting strength', 'strong lifts 5x5' and 'strong curves' are some good beginner programmes.

I agree with the other replies. Just want to add that weight training helps raise your metabolism and burn calories all day long, while aerobics will burn fat and use up water reserves of the body during exercise mostly and much less afterwards. Thus, for firming up the "flesh" as you lose weight (toning), weight training is superior to running, since it prevents "hanging flesh" after fat has been shed. Depending on how fast you react to weight training, you might consider using the mirror (how you look) and your clothes (how loose they now fit around the waist) more than the scale to assess the success of your training and diet combined. And do not worry that you might start looking masculine or beefy...Considering women have twenty times less testosterone in their organism than men, becoming "muscular" as a woman is extremely difficult. Enhancing your natural femininity and beauty is a snap. Good luck and enjoy!
PS: In a previous reply someone mentioned "macros"... In case you do not know, that person means the ratio of macronutrients within your daily food intake, namely, the percentage of calories from fat, protein and carbohydrate within your total caloric intake. For example, if you consume 1200 calories daily, if you want to eat lets say 34% fat, that would be 400 Calories (400/1200 = 33.34%) and since 1 g of fat contains 9 calories, that would represent 44.5 g of fat eaten that day among the total number of meals eaten that day. Which leaves you with 1200 Cal - 400 Cal = 800 Calories. That's 66 % of your calories left. Then, you must decide how much protein you might want to consume a day. For the purpose of this explanation, let's suppose that from your dietetician's advice, you would split your protein to carbohydrate intake: 1/3 P and 2/3 CH. Thus, 1/3 or 33.3% of 800 Calories is 267 Cal from P and you're left with 533 Cal from CH. Since 1 gr of Protein and 1 gr of Carbohydrates both contain 4 Calories, you divide 267 by 4 = 67 g of protein, and 533 divided by 4 = 134 g of Carbs. In summary, you would then consume roughly 45 g of fat daily, along with 67 g of protein, and 134 g of Carbs, divided among 3 to 6 meals per day. Thus, you overall macro would then be: 33% Fats, 22% Protein and 45% Carbs. Using an Excel spreadsheet helps a lot! Hoping this clarifies "macro-calculations", I wish you great success and vibrant health!