Straightening hair once a week wont stop your growth. You should use heat protectant (whether it helps or not is a whole other convo)
Ceramic irons only...
Just understand that you risk heat damage if you use too much heat. Everyones hair can only take a certain amount of heat. What my/your hair can take and not be damaged is going to be different from everyone else- but its not worth experimenting either. I would suggest not putting your ceramic on high.
As far as keeping it straight- wrapping your hair.