As you say, your biggest challenge is to avoid doing too much at once. Exactly what is 'best' is going to be subjective for the most part, so what I'm going to do is give you a few stages you can set goals at and work toward without getting too overwhelmed.
On the diet side:
- Stop eating fast food. This will break you of some of your bad habits and provide a good basis for a new routine.
- Stop eating out completely, unless it's socially. Learn to cook and prepare your own meals. This will establish part of the routine that Aaron was talking about.
- Read up on preparing diets for specific fitness goals (or post more questions here when you get to that point). This will empower you to cook for whatever your goal is, and give your routine direction.
- Learn about various ingredients and how they affect your nutrition (or, again, post more questions on ingredients you don't understand). This will allow your cooking to satiate your appetite better, and give your routine greater effect.
Once you're informed, and you know enough about your food that you can prepare meals that are very satisfying, you will never want to go back to a McDonald's again. You will know for a fact that your own food is both more healthy and more satisfying than the junk they pedal there. Your routine will work for you and it will be effortless to stick with it.
On the fitness side:
- Spend 15 minutes outside every sunny day. Read a book and enjoy the sunlight. You don't have to start with any particular exercises. Just stroll through the park, around the block, sit on a bench or your lawn and read. This will start to break your sedentary pattern and provide some (probably much-needed) vitamin D to start activating your body. You will also start to associate this time with leisure and fun - which is what your exercise should be in the first place. Do it on your break from work if you want.
- Turn those 15 minutes into a brisk walk. Don't push yourself or try to break a sweat. Just walk a little faster than you normally would and breathe deep. This will start to establish the exercise routine.
- As you get more comfortable, extend that 15 minutes into 30-45, whatever's comfortable. This provides the basis for the exercise period you'll work into. You may have to move your time slot to make it fit.
- Start exploring (more questions here would be great) on what sorts of exercises are out there, and what sort of exercises interest you. Exercise for the sake of exercise is not sustainable for most people. Most people need to exercise for the sake of fun. Ask around for input until you hear something that makes you say, 'hey, that sounds fun!' Then work that into your normal workout. You will want to start with cardio exercises since those tend to be easiest and provide the biggest mental boost.
- Start learning how to work toward weight and strength goals. Again, more questions here would be great, as would be help from fitness professionals. Learn a structure for stabilizing, isolating, and strengthening muscles so you can fulfill needs when you see them. This will really empower your routine and make you feel like you're absolutely in control of your body.
Once you've gotten to the point where you mostly know what to do in order to make yourself feel and look better and stronger, and your exercise is in the context of being fun leisure time, you will eagerly look forward to your workouts and feel annoyed when you have to skip them. At this point you have a very sustainable situation where you no longer have to put forth effort to exercise; you'll just do it automatically.
These goals should be hit and surpassed as you feel comfortable. Breaking yourself of fast food may take 1-2 weeks, and that's perfectly fine. You may want to completely master the kitchen before you touch your workout. You may want to tackle both simultaneously. It may take a year or more to really change everything. There's no good pace or bad pace, fast or slow. It's all up to you, it's the destination that matters. The journey is all yours.
Hoping to see you around here more often (and with a permanent account) so we can help you work out the particulars when the times come!