There are so many different options for signs that it can be complicated knowing where to start. For any sign though, it is important to think through its exact function. Once you know and understand the exact purpose of the sign, it is then easier to develop it.
When designing other signs, it is important to bear in mind the colour scheme. Many colours already have universal connotations such as yellow and black or red and black symbolizing warning or danger. If this is not the image you are trying to promote, then it may be best to avoid these colours.
Similarly, sign shapes do play a part in what message the sign is giving. Round signs tend to be associated with prohibited actions and triangle shapes are usually a warning. For everyday use, it is safer to stick to a simple rectangle.
No matter the sign, text always needs to be readable, and it is always suggested to add a graphic or image of some kind to simply break up the text and provide an alternate method for conveying information. Sign size will also vary depending on whether it is to be displayed internally or externally.