Start with a few easy exercises that you feel you'll be able to do for 2-3 times a week. It doesn't have to be much, doing something is still better than doing nothing. If you manage to stick to this easy routine for a few weeks, try adding other exercises or a more challenging version of what you already do. Keep doing that until you're satisfied with your routine :)
Two tips from me: Have a big, big "WHY" and by starting small!
By 'why' I mean the reason you want to exercise. :) Be as specific as you can about it! Is it to lose weight? To run a marathon? 'Why' do you want to lose weight? To reach a more healthy BMI? It's important that you be specific so that this will serve its purpose! Trust me when I say that it's really, really easy to fall out of your exercise routine the moment your motivation runs dry. What's left when you're out of motivation? Your 'why' acts as your pillar of strength, your reminder as to how important this routine means to you! That, and you can then research around your goals to find out what type of sport or exercise regimes that work best to achieve your goals. :)
2. Starting small.
Before you sign up for that gym membership, or before you tell yourself that you're going to wake up at 6 a.m. every day to jog for half an hour in the morning, slow down. Start really, really small. If you're starting off from completely no form of exercise, take it easy. Don't jump from 0% to 200% at once! Try running once a week, and take it from there; or try doing 5 push-ups a day, then build it from there. 3 days later and 5 is too little? Make it 10! 10 becomes easy too, how about 15? Be consistent about it! Only go to the next level when you're sure you're comfortable with the current level of exercise. Transitioning into a habit takes time, and if you overexert yourself by going from 0 to 200 in just a snap of a finger, the routine will be extremely difficult for you to build because you'd be demotivated by how little you're achieving compared to what you expected it to be. If you start hating your exercise regime by the first few weeks, it'll be near impossible to make exercising a habit, because habits take at least 3 months to form! And remember, exercise is supposed to be a lifetime activity, so take it easy! You have all the time in the world to slowly build a routine that works for you. :)
Hope this helps!