Hi Haley,
I am a 23 year old male who stands 5"11 and weighs 160-165 lbs.
I don't know what or if there even is a standard weight for my age/height and what that falls into, I certainly know I'm not large by any means.
I do however feel "pudgy" compared to my 20 year old self who could eat what he wanted and not feel the repercussions. Back then I stood the same height and about 150-155 lbs and I was much happier with my body then.
I try and eat healthy but it's not always easy and I end up making quick dinners such as pastas and or other carbs a lot of the time. I enjoy eggs/oatmeal/fruit for breakfast, a sandwich for lunch, and some protein at dinner that is not red meat plus carbs and veggies, but it's not always easy to prep these things.
I walk about 30 minutes to and from my 9-5 job and tend to do a lot of walking in general, but that is the extent of my "exercise regime".
I want to know how to slim down a bit and tone up, no big muscles or 6 packs or anything like that, just be more defined and lean.
Please help!!

According to your Body Mass Index (which is a general way of determining if your weight is healthy based on your height and age) of 23, your weight in considered just below the average for your height and age.  However, this number does not tell us how much of your weight is fat, how much is muscle and how much is your bone weight.  Rest assured you are not overweight but I understand your frustrations in how you no longer look like your lean 20 year old self.

We all use the excuse that our metabolism decreases as we age but according to the mayo clinic our BMR (Basal Metabolic Rate, which is how many calories our bodies need to do necessary things like breathing, digestion, etc) stays relatively constant our entire lives.  What changes are our muscle mass and our activity levels, which will decrease the amount of calories we require in a day.  If we do not adjust our caloric intake we start to gain weight.

Here is what I think is happening with you: you are not moving as much as your 20 year old self used to and when you do move it is not at the intensity that it used to be.  Because of this you have lost some of the lean muscle you once had and it has been replaced with body fat.

The good news is because you are young you should not have a problem getting the body you desire. Based on your weight I presume you have always been slender so understand that you will have a very difficult time looking like Hugh Jackman in X-Men.  Instead think more along the lines of Ryan Gosling.

Muscle does not build itself so you need to start building it and that means using your body to lift things that make you tired and sore the next day.  Whether you are lifting weights in a gym or doing push ups and lunges every morning and every night you need to do enough of them that your muscles are screaming.  To build muscle size (which is called hypertrophy training) you want to do 3-5 sets of 12-15 repetitions, making sure that the last repetition you do is the last one you can do with proper form.  If you have time to work out everyday then I suggest you split your body up and focus on different parts each day.  Monday and Thursday maybe you do all the pushing exercises: chest press, dumbbell press, incline/decline bench press, tricep dips, tricep pull downs, etc.  Tuesday and Friday you do all the pulling exercises: lat pull down, bent over rows, pull ups, bicep curls, etc and Wednesday and Saturday you do your legs, shoulders and lots of core exercises. 

If you do not have the luxury of going to a gym everyday then pick 3 days a week where you do 1 or 2 exercises for each part of your body and throw in a core or cardio exercise between each set (burpees, planks, mountain climbers). A good option is looking into the home workouts, such as P90X or the Insanity Workouts.  Also check out this site as it is full of information that you might find helpful.

You also need to get rid of the “pudgy” fat that has crept onto your body and this is done with cardiovascular training and a clean diet, in combination with strength training. It sounds like your diet has a high concentration of simple carbohydrates and as long as you are putting those into your body you will continue to keep that soft layer on your body.  I am not saying cut out all of your carbohydrates (I am a big believer that if you are active you need to eat carbohydrates) but they must be healthy and complex carbohydrates so choose things like vegetables and whole grains (quinoa, kamut, barley).   No more pasta and no more bread. It sounds harder than it is but with a little bit of planning and experimenting with new foods you will find you will actually enjoy eating this way.  Remember, 6 packs are not built in the gym, they are lost in the kitchen!!

For your cardio workouts I suggest short and intense sessions.  Try doing intervals on a spinning bike or sprints at the track.  You really want to go for maximal calorie burning with minimal time so you do not have to spend all day in the gym.  In simple terms, get your heart rate up and sweat. Pack some running clothes with you to work and before you head home go for a 15-20 minute “Fartlek” run.  What is Fartlek? It is a very easy way to do speed work as all you have to do is pick an object in your vision and increase your speed until you pass that object.  Slow down, recover and when you feel ready to go again pick something else to sprint too.  The farther away the object is that you choose will determine how fast you run.

Get on top of this now and it will be easier than you think but if you keep living the lifestyle you are your 30 year old body will be wishing you would have started taking better care of yourself a long time ago.
Good Luck! 

Attached - David Duchovny, not 23, heading to the gym.