Don't listen to those people--they have a terrible attitude. Everyone should care about their health, good for you for recognizing that!
I can relate to you, although I wouldn't have considered myself 'too thin', just slender. I was actually sort of 'skinny fat'--small looking, but still a lot of jiggle! I didn't work out much and ate whatever I wanted, but I wasn't living a very healthy lifestyle and it would have caught up with me eventually. In 2008, I decided I wanted to be healthier and obtain a more lean figure (more muscle, less fat--this is actually what the so-called 'toned' look really is).
I started with eliminating obvious junk from my diet/lifestyle--I quit smoking, dramatically cut down on my fast food intake, completely eliminated all beverages except for black coffee (no more than 1 or 2 cups a day), water, and tea (and the occasional V8--no fruit drinks, they are too high in sugar and full of calories, and you're better off just eating fruit). I also started taking a multi-vitamin and fish oil capsules. I also try to limit my drinking to once a month, if that, and even then I drink red wine (NO mixed fruity drinks, those are calorie and sugar NUCLEAR BOMBS).
I steadily started improving my diet by choosing whole grain over white (like with breads), eating more vegetables and fruit, and choosing lean meats (like chicken breast, or tuna). I read all labels on food now, and even keep track of my calories in a book, but that might be too obsessive for some (that's just how I like it!). I eat a lot less processed food and refined carbs. The more 'whole' your food is, the more full and satisfied you feel, and the better it is for you.
As for exercise, I started walking more! I walked for about 40-60 minutes every day, usually spread out over the day. I also started to lift weights--don't be scared of this, a lot of women are and it's unfounded. You need a strength training routine if you want to add muscle--you won't look like a man, just toned, and more muscle=more calories burned and a higher metabolism. However if you are 'very thin' you may have to eat more than you are used to to achieve this.
A good resource to check out is
Free Diet Plans at SparkPeople. They have nutrition and fitness articles that I've found very useful, an exercise database, an online community, and more! If you sign up, you will get your daily calorie requirement calculated for you, and macronutrient ratios (carbs, fat, protien).
When I first started, I followed a basic routine, like this one:
The Bikini Workout
You will probably want to start with the beginner strength program. Don't worry about it saying it's for weight loss--you can use it to get healthier and more 'toned' too, maybe just eat a bit more. It's a good way to start. The diet is good too because it shows you the proper macronutrient ratios (fat/carbs/protien).
The Bikini Diet <-- link.
There is so much to know, you can start reading the articles on the site to learn more. Don't feel overwhelmed, though--you can start slow using the tips I mentioned earlier.
Keep in mind too that it's a lifestyle change, not a quick fix or something temporary. Work up to things slowly if you can, it can take a while to change your habits. Good for you for taking charge of your health and body! I found it very empowering, good luck!
. I'm 5'4" and 113 lbs.
