No two depressions are the same (not even for those experiencing several in a lifetime) so it's hard to say exactly how it expresses itself...
This.
Depression can affect anyone of any type and manifests in many different ways. Introverts appear to be more prone to falling into deeper bouts with it if only because they are more likely to isolate themselves and equally less likely to ask for help (neither of which are very good strategies when dealing with a mood disorder).
In general, the best thing you can do is make sure you are taking care of yourself physically. The most important thing is getting enough quality sleep on a regular basis and avoiding naps during the day. It's a good idea to have a bed time and waking routine that you stick to consistently; in bed at the same hour every night and out of bed the same hour every morning.
The next order of business is getting your nutrition in order. Make sure you eliminate or at least limit the amount of caffeine, sodium, sugars and processed foods you keep in your diet as these tend to exacerbate depressive symptoms in the long term. Stick to healthy, nutritious foods. More veggies, fish, poultry, and 3 or 4 pieces of fruit to keep your blood sugar steady throughout the day. Supplement with good quality omega 3's and vitamins
Thirdly, regular moderate exercise and daily fresh air. This really does wonders and exercise is a natural mood lifter! Be sure to get at least 30 minutes of it each day or every other day.
Lastly, or perhaps firstly, you should go talk to your doctor. Ask for a blood test to make sure you're not deficient in some key vitamin or mineral and that your thyroid is in order. If you're female, it may also be a good idea to monitor your cycles (if you do not do that already) and get checked out for PCOS (Polycystic Ovary Syndrome) which is very common in many women and a frequent cause of depression.
Once all your physical basis are covered, and you got a nice, healthy vessel, focus on your psyche. Make sure you eliminate any unneeded stressors. Cut ties/limit interacting with any people that bring you down and focus on the positive, excitable people in your life and regularly schedule to see them. Let their good mood rub off on you. Do not isolate yourself. If you do not have many positive people in your life, I would suggest signing up to do some volunteer work once or twice a week. The act of helping others and interacting with new people who are kind and like-minded will definitely keep the ruminating thoughts at bay. If you have the money, I would additionally suggest signing up for a class you always wanted to take at a local community center and just basically, FILL YOUR SCHEDULE and LIMIT THE AMOUNT OF TIME YOU SPEND ALONE.
As for when you are alone, be sure you give yourself an outlet to express your feelings. If you need to cry, cry. If you need to scream, scream. If you need to just vent, call up a trusted friend or record yourself as you let it all out. Or write it Or draw it. Whatever. As long as those feelings are out there, you're good.
The key, however, is to limit the amount of time you spend ruminating on all this stuff and searching for answers. The reason why I say this is because depression brings with it a lot of inertia and the longer you stay at rest, the harder it's going to be to get moving. It is vitally important to at least have one foot in the regular world doing things that are active and pleasurable and social (even if some days you don't feel like doing any of it) while the other is in your private world working through the conflicting feelings. Otherwise, you'll find that there really is no end to the rabbit hole. Some people spend years indulging their depressive feelings full time, searching for 'what's wrong with them' and 'why did this happen to me." This rarely leads to recovery. The nature of the human mind is going to only come up with more problems, more issues, while the rest of the world keeps spinning and you fall further and further behind because you keep ruminating on the past and sooner or later, you're going to have more reasons to feel down about yourself because you haven't been moving forward on your goals.
The key to beating depression is movement. Movement on your goals, movement out of your comfort zone, movement out of the ordinary, and of course, physical movement. If you're standing still and giving into your negative thinking instead of living your life, you're only adding more fuel to this black little fire.
Take it easy on yourself, but also make sure you're not going too lax. Depression has an insidious agenda. It wants to convince you to do less and less so you can pay it more attention and let it fester. Get acquainted with your limits. Don't bite off more than you can chew, but also don't underestimate what you're capable of either. Ask for help. Talk to someone. Take care of yourself.
