Complex Carbs (e.g oats, beans, potatoes, brown bread brown rice and brown pasta) are the ones you SHOULD eat. Simple (i.e sugary carbs; white bread, crisps, pasta, rice) carbs get broken down fast and the body stores it as fat if you dont use it quicky, so only eat simple carbs pre workout for energy (if you need it) and post workout to replace glycogen in muscles. Like 50 grams id recommend. These will influence if you gain or lose weight most of all. Complex carbs also fill you up so will also prevent you overeating as they take longer to digest due to the fiber.
Eat as much veg as you want for minerals, fruit is ok slightly sugary though so try and limit this.
Fats like peanut butter and nuts are great and also some saturates (in moderation) (go for about 60-90 grams of fat in total a day). Try and keep it unsaturated if you can. Saturated fats are in animal sources, unsaturated is in non animal sources like oils, nuts etc. Fats are essential for hormone maintenance too.
As for protein, all meats and fish are great. Dairy products are great. Basically try and eat as natural as you can and keep your protein intake at 1.2-1.5 grams per lb of bodyweight per day if you want to maintain and build muscle, its needed for when you are weight training to repair the muscles too. This will help with the muscle gain and definition you are after. If you look slim but also flabby chances are you arent eating enough protein.
Drink 1-2 gallons of water a day.
Optional but really helpful: Multivit tablets, Fish Oil tablet.
I also use this as a guideline
http://www.freedieting.com/tools/calorie_calculator.htm
*<50 grams of carbs per day for rapid weight loss along with the calorie deficit intake provided by that calc. You NEED to keep your protein high (1.2-1.8 grams of protein per lb of body weight) for this or the body will break down muscle tissue for energy. Eat lots of fats (steak, peanuts etc) for energy also. This is the one im using at the moment to strip myself of bodyfat and get definition. Not recommended for prolonged periods.
*50-100 Grams of carbs per day for gradual weight loss along with the calorie deficit intake provided by that calc.
*150 Grams of Carbs maintenance. See calc for calories
150-200 grams: gradual weigh gain. See calc.
*250+ grams. You'll either pile on weight and become obese or you run marathons lol.
I find the rule of thumb with carbs is if you are an ectomorph and/or do prolonged duration exercise or stupidly intense exercises and are more active you need more carbs and obviously more calories. This also applies if you are lean as you have less fat stores to use. If you are en endomorph or mesomorph i.e are more prone to weight gain, you need less carbs and calories. Again all of this will be determined on your activity level so its best to find what works for YOU. Some people also have a bad metabolism which also needs to be taken into account.
If you want to bulk up or build muscle obviously you need carbs again based on everything above and including a calorie surplus. (Add 500 onto what the calc above provided). If you are new just shoot for maintenance calories and just make sure you are eating right as you will make gains regardless. When you plateau then you should bulk up after maybe 1/2 years+ of weight training.
Remember, the key to weight loss is insulin (and of course calorie in vs calories out). Insulin is activated by carbs. Insulin is a "storage" hormone, it either stores carbs as glygogen in the muscles/liver post workout or as fat any other time if you dont burn it off. TL;DR if you are eating a diet rich in carbs and arent burning that off every day, you are going to have a horrible time with weight loss see: most of the population these days lol.
http://getinspiredgetfit.com/picture/473/carbs-are-killing-you!/
http://www.marksdailyapple.com/a-metabolic-paradigm-shift-fat-carbs-human-body-metabolism/#axzz2YaZznJwr
Interesting reads for when you have the time