Anti-depressants usually take weeks before you start feeling any benefit. I have always found it is better to take the medication immediately before bed time. This way, any drowsy effect you might feel when you first start using them, wont affect your day to day activities. After about a week or two usage, it's important never to just decide to give up and suddenly stop. They are not addictive, but can play havoc with the chemicals in our brain if we stop suddenly; it's always best to keep in touch with your GP and trust his guidance.
Anti-depressants react differently for each individual, it's difficult to be specific about possible side effects. Some people, like me, have almost no side effects from any of these meds. The patient leaflet that comes with your medication is good to read and there will be loads of results if you type 'Sertraline' into a Google search.
You're Mum is right in one sense.... yes, they might change you.... but, for the better. Hopefully, combined with therapy, you will start to feel an improvement in your mood in the coming weeks. Check in with the people on this Forum when you need any advice or support
Good luck $%$
