Do you want to display text like in a dialogue or would you rather have a sign with a custom text already written on it?
1. If you want a dialogue to pop up, the fastest way would be to just put your sign in the world and then create an npc with all the dialogue you want to be displayed. In the main window of the entity of the npc, click on the meshes (they are called something with "head", "legs", etc.) and go to their properties, where you can set "drawableFlags" to invisible. Now you have an invisible npc you can click on.
You can also create a new entity template, make it a NewNpc and just let it look like a sign, but I am not sure if it works.
2. You can look for a sign mesh you like and find the texture its using. Now rightclick on this texture, click on export (sometimes you don't see "export", so you have to click on "Reimport Resources" first) and you can then edit this .dds texture outside of Redkit with a program like photoshop. Now save it and in the asset browser click "Import". Find your texture, import it (as World Diffuse) and then copy the sign mesh you liked and assign the new texture to your copy.
It is important that both the texture and the copied sign mesh are stored inside your level folder. Store everything you edit in the asset browser into your level folder.
In the entity of the npc, click on all mesh parts for the body, the legs, the head, arms, etc. and set them to invisible, not only the character entity. Then it should work. Or create a new appearance and just don't select any body part.