I've just created my first photoshop image however as I was saving it, photoshop said that layers will make the file size bigger. I don't want this to happen so any help will be appreciated!
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The above answer is correct, if transparency is not important to you. Once the image is flattened, it will have a solid (white by default) background colour. As you did not state why you chose the TIFF format, one can only guess what answer would best serve you.
If another format is okay, and you want to maintain transparency, you can go to
File » Save As and choose PNG. Or
File » Save For Web and select PNG which will give you more control over the colour palette and file-size. The large size of TIFFs make them well-suited for high-resolution print graphics and photography, but not most beginner tasks.
Whichever option you choose, it is important to
Save As a Copy if you are going to flatten the image, so that you preserve the layers and keep them editable.