You've already made the decision to work out so you have the motivation, you just need to feed it. Just keep thinking about those reasons you already have.
Few tips - the first week is always the hardest and because of the pain and tiredness it makes it incredibly easy to throw in the towel straight away. Push through because once you get used to it the hurtful pain goes away and then youll notice there'll be a new kind of pain, an enjoyable one.
Make sure you have your diet worked out, without it you will see no results.
Don't be a pussy, stick to free weights as much as you can.
Get a gym buddy or at the very least get people at the gym to spot you, you will gain much quicker with some guided assistance and even quicker with a buddy to keep your motivation high.
I started out at a measly 60kg, didn't look good. It's an advantage though because any muscle gained is seen straight away and definition goes much farther than size. I packed on 15kg of muscle and it completely transformed my entire life. I received more respect from strangers, felt more comfortable and confident, received a HUGE amount of attention from girls - for the first time ever I had my pick AND I had a lot to choose from :Feels good man: it's a different life being fit, once you experience it youll be a changed man. Seriously, doors open and opportunities just appear for you.
To help me keep on track I go to the gym after work as I have more energy then, morning sessions? Fuck that. No energy. Also get onto this, it'll make you want to work out, good stuff
http://www.bodybuilding.com/store/clabs/flood.html
Check out my posts from the nudie thread if you wanna see how I transformed myself and hit me up if you want a good training routine to follow and diet too. I did all the hard work already and worked out a good 4000 calorie per day diet made of 40% protein, 40% carbs and 20% fat (very fast metabolism here). My diet is decent on the budget as well, running at around $250 per month for all my meals.
Good luck man!