You need to be prepared for it to take a long time. In the region of 2 to 3 years. If you try and do it more quickly you will fail on many fronts.
If you benefit from peer support a slimming club can be useful - but as with all these things a lot depends on the skill and personality of the group leader.
Online groups - such as Weight Loss Resources, can be good (and cheaper than a physical group) if you respond well to keeping a diary. With them you will record all your food (and calories) - it can also be motivational.
I would suggest for the first couple of weeks you don't try to reduce specifically, just record everything that you eat. That way you will be able to look at your diet and work out changes that you can make that suit you. Weighing all your food is very useful too. Sometimes, when we are used to comfort or mindless eating, we lose sight of what a normal portion is.
Take it steady, be kind to yourself and build little treats in - but don't keep in the house anything that you are unable to resist.
I love crisps and can easily eat 2, 3 or even more packets in a day. I don't want to totally deprive myself so I keep multibags in the boot of my car. I can then have I bag if I really want to but I am not just mindlessly grabbing a packet as I walk past or because I can't be bothered to make proper food.
You will learn what your triggers are, what you can give up and what you can just cut down on.
The good news is, if this has been one journey up to that weight and you haven't yo yoed over the years, losing weight will probably be easier as your body hasn't coped with restrictions before. If you have gained and lost a lot before it's harder but it is still definitely doable. And even if it's slow. Every pound is one in the right direction.