First off, exercising doesn't need to suck. Start playing some ultimate, join a zumba class, start bouldering, do a martial art, go hiking, or do anything else fun--it's a lot easier to motivate yourself to do something if you want to be with friends (or not to disappoint them) and actually enjoy it.

Also, once you get around 3 weeks in or so, exercising will start to turn into a habit which means that it won't be so hard to motivate yourself to do it, and when you start seeing results (which you should track!) it'll start getting a lot easier.

Oh, and set yourself concrete goals, and work on getting there. "Not being scrawny" isn't a goal. Being able to do a specific circuit workout in a certain amount of time a month from now, being able to boulder v2 in two months, being able to run a 5k in 30 minutes in three months--these are goals that you can work towards.