If there is only one salad on the menu, you could say either "I'll have the salad" or "I'll have salad". If there is more than one salad on the menu, you would have to specify which one - "I'll have the Caesar Salad", "I'll have potato salad" etc. In BrE, we don't use "take" when ordering food or drinks in a restaurant.
You will hear "take" used frequently by non-native speakers, usually because it's a direct translation from their own language. For example
Je voudrais prendre un café au lait (French) = I would like to take a white coffee (coffee with milk) - Direct translation
Me gustaría tomar un café (Spanish) = I would like to take a coffee
Prendre = to take
Tomar = to take
In English, it would be "I'll have a coffee" or "I would like [to have] a coffee".