The first thing you need before a diet plan can be planned out is to find out what your caloric needs are based on your current height, weight and age. Start by going to this site and plugging in your information to find your current BMR (Basal Metabolic Rate).
As an example I will use my stats. I'm 33 next month, 185lbs, and 5'11". My BMR came out to ~1896. This number is essentially the amount of calories my body burns with an activity level of 0, or completely sedentary. Now I will apply the Harris Benedict Equation to my BMR which will determine how much I truly need to consume in order to maintain, I say again, just to maintain my current weight.
If you are sedentary (little or no exercise) : Calorie-Calculation = BMR x 1.2
If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR x 1.375
If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR x 1.55
If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR x 1.725
If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR x 1.9
I am a very active person with a very high metabolism. The rate my BMR x 1.9 doesn't cut it as I know I will begin losing weight. I work 5-6 days a week and am on my feet most of the day. I can walk upwards of 5 miles easy on one shift. I also ride my bike to work and back almost every day with that trip being ~6 miles each way. On top of that I lift 3-4 days a week for about an hour and a half on average. Therefore I use my BMR x 2.5 (1896 x 2.5 = 4736).
Now I don't eat 4736 calories every day. Sometimes I eat a bit less on days that I don't work and it's not a day to hit the gym, and other days I will eat more when the need arises. It will be up to you to figure out the balance.
The key is to keep your weight gain to about 1 pound a week. Lean muscle will be hard to get as you will inevitably gain a bit of body fat in the process, so the key is to go slow.
Once you figure all this out, then we can get into more details on what to eat, and when to eat.