Spring garden prep?!

Hi!!! Our garden is this sort of L shaped space enclosed by the wooden borders. Last year we had tilled the whole space—then grass grew through & around everything & now it’s basically just yard again 😅 I will start seeds indoors soon but will need to make sure the garden is ready for transplant time. How do we prep it? Do we fill it in with topsoil/etc.? Do we need to till the grass out? What sort of barrier should we use (if any) to keep grass from growing?

Thank you so much!!