need help because have a big school project
You'll need to use a shader that uses alpha blending in one form or another. (Note that there are multiple ways to specify and modify alpha values, such as specifying them in the texture's alpha channel, specifying them at the vertex color level, and so on.)
I actually did something like that recently. What I did was made the texture in GIMP. I went to layers>transparency>add alpha channel. In the layer toolbar, I set the opacity to what I wanted it to be. Then I saved it as a .gif and sent it into Unity. In the texture dialogue for it, I checked "Generate Alpha from Grey-scale". Finally, I added it to the object, set the shader as transform diffuse. If you need more help feel free to ask.