Wagamama have a pretty cool menu on their website with nutritional info for each dish, so finding something healthy to eat should be pretty easy.
Although a salad might seem like a sensible option, something like the Mandarin + Sesame Chicken salad actually contains just as many kcals as a ramen dish.
I'm not a big fan though, and it's only a side - so you'll still need something to fill you up.
Personally, I think most of Wagamama's menu is fairly healthy. You should be good with most of the Ramens really. Some Japanese food enthusiasts find the menu a little bland, but in terms of healthy eating, something like the house-special Wagamama Ramen ticks all the boxes:
Fatty meat like pork or duck is a no no.
And any of the fried dishes also aren't a good idea. So no gyoza (fried dumplings) for starters, and definitely nothing like the Chicken Katsu Curry (which contains deep-fried breadcrumb chicken).
If you're still struggling, just ask the server - I find most Wagamama staff very knowledgable about their menu.