My position seems to agree with most seen here so far, but here it is anyway.
*100kg on a 5'9 frame is fat and is unhealthy. Thus you should loose weight for your own health.
*Clearly this is hurting your self-confidence/self-worth. Thus you should loose weight to feel good about who you are.
*Physical attractiveness is an important part of whether people want to have relationships with other people and the tendency is for relationships to form between similarly attractive people. Thus if you want to date (date as opposed to fuck, the later just requires more availability) more attractive men then you must be more attractive loosing weight will go a long way to doing this.
Ok so we've established that you should loose weight now here are the caveats:
*Loosing it to win back an ex will fail and is a bad motivation.
*Trying to loose it too quickly will be bad for you (especially once you pass the initial period of easy loss).
*DONT become anorexic or bulemic.
*DONT loose motivation because you fail to loose a kilo a day for a month and transform yourself into the hottest girl at school in a month. These things take time and effort.
And now some practical advice on loosing the weight:
You must eat healthily and exercise.
You will likely need to eat less and exercise more.
As far as diet you should:
Eat: lean meat, chicken, eggs, etc. Minimise the ammount of frying. Avoid refined sugars (eg fruit good candy bad). Go for higher fibre carbs eg brown not white rice and bread. Eats lots of vegetables.
If you live at home this may mean cooking your own meals to avoid your parents unhealthy cooking or persuading them to eat healthier.
As far as exercise you should:
Do lots of cardio, especially; riding, rowing and swimming.
Where ever possible substitute walking/riding for driving/etc.
Do some bodyweight exercises (crunches, pushups, tricep dips, hyperextensions).
Do some weights; free weights and compound movements (squat, deadlift, bench press, chin-ups).
Take up a martial art; loose weight, build flexibility, gain confidence and defend yourself.
Joining a gym would be a good idea.