When it rains, it pours, I guess. Second texturing thread in as many days here on the building forum that should be on the texturing forum. Anyway...
From: GeoMan Pinazzo
1) How do I place text on the rectangle to look a sign
I'm not sure what you mean by "the rectangle" (which rectangle?), but as Alice said, what you want to do is learn how to make textures. You can use any image editing program for this. The most popular are Adobe Photoshop, Corel Paintshop Pro, and The GIMP. I recommend Photoshop if you can afford it.
For something as simple as text on a rectangle though, really, you can use just about anything. Even MS Paint will do the job. Simply create an appropriately sized canvas, grab the text tool (all image editors have a text tool of some sort), and start typing. When you're done, save the file and upload it to SL. Then slap it on a cube.
See the sticky at the top of the forum for information about what canvas sizes and file formats you can use.
From: GeoMan Pinazzo
2) How do I add more textures which folder is it or where do I get them?
To add more textures, you upload image files from your hard drive to the SL servers. These can be images you've painted yourself, photographs you've taken, pictures you've downloaded from the Web, any appropriately sized and formatted digital image file will work. Just be sure for any image you didn't create yourself, that you do have appropriate copyright permission from the image's owner before you use it. Also, do yourself a favor, and read the sticky mentioned above before you upload anything. There are some important things to know about sizes and formats.
To upload, the command is File -> Upload Image if you're using the standard LL viewer, or Build -> Upload Image if you're using the OnRez viewer (not sure about other viewers). Newly uploaded textures will appear in your Textures folder in your inventory. It will cost you L$10 for each file you upload.
You can also buy textures in-world from other residents. But as with Web images, just be sure to adhere to all applicable copyright law (i.e. don't resell or redistribute any images without the author's express written consent). The in-world permissions system can be a little deceiving where copyright is concerned. Just because an item might be marked as able to be given away doesn't mean you actually have the right to do so. Abilities and rights are two different things. As someone recently said in a thread on this topic, we all have the ability in RL to copy CD's and resell them, or to photocopy books and resell them, but we all know that's illegal, so most of us don't do stuff like that. Treat other people's textures the same way. If you don't know with absolute certainty that it's OK to redistribute something, don't do it.