It is a good idea to get your bitch spayed. This will prevent her coming in season and prevent any possibility of mammary tumours or pyometra, and of course will stop her getting pregnant.
Provided you feed her correctly and give her the required exercise for her breed then she should neither gain or lose weight. It will not make her ill.
Ask your vet when he prefers to do them, as some vets like to do them before a first season at about 6 months, and others like them to have had a season first. They should then be done about halfway between seasons, i.e. after about three months since the last season. I have had experience of both, the bitch spayed at 6 months showed no ill effects, and others done later have also been fine.