We all have lazy days and sometimes I feel lazy more often than I would like. To be more motivated to exercise and go outside, the important thing is to make working out a habit, something that you do daily or very often, so that your body gets used to it. Create a routine that you do daily, so you can stick to it. You'll notice yourself flying through your routine faster by the day and feel less and less tired, which shows improvement. My routine consists of lunges, squat, sit ups, and then either running, biking, or swimming.
Once you start, you won't want to stop! Healthy is a lifestyle that you have to live. It's a commitment, but when you're older, you're going to appreciate that you spent time of your day to exercise. I'm sure you'll want to pat your younger self on the back (not possible, but this is just metaphorical:P ) for making good decisions!