The following is intended to give you sufficient information to produce a simple poster.
File/New or Ctrl/N. For A4 wide (landscape) set Width: 29cm, Height 20cm. For A4 tall (portrait) set Width 20cm, Height 29cm. For A3 wide set Height: 29cm, Width:40cm; for A3 tall set Height: 40cm, Width:29cm. Set resolution to 100 pixels per inch.
Click on T in toolbar. Click anywhere on canvas (graphic). Text box appears. Enter text or paste into it from another source and then select it with the mouse to set attributes like size, font and colour. Click OK to confirm.
Choose Move tool or hold down Ctrl key and drag text into position required.
Avoid large chunks of body text unless they are for artistic purposes (part of the design, not to be read).
Each time you press OK you create a new text layer and you can end up with a dozen or more unwanted empty layers. Check the Layers palette for unwanted text items (mistakes). Drag an unwanted layer onto the dustbin icon at the bottom right of the layers palette.
To make text look 'distressed' do the following:
For a more controlled effect:
This can provide a good effect for posters.
You may want to use pictures and images from web pages. Right click an image on a web page and use Ctrl-C to paste it into Photoshop. Each new paste operation creates a new layer so you can move images around.
Become familiar with the Layers palette. Leave it in the bottom right corner and don't switch it off. You may not need to add new layers but you will certainly want to delete unwanted ones - drag them over the dustbin. To merge two layers click on the upper of the two and press Ctrl-E (Layer/Merge Layers). To merge all layers choose Layer/Flatten Image.
Use the dotted rectangle tool to draw around the part of an object you want to preserve and then choose Image/Crop to remove the surrounding material. For any object you want to place on a larger canvas do the cropping first and then copy and paste it.
Use these techniques carefully and sensibly. You want people to like your images, not to laugh at your silliness.
To change the range of colours in an image press Ctrl-L (Image/Adjust/Levels/) and drag the triangular sliders. Reducing colours can produce dramatic effects, changing the contrast. This can work in layers or across the whole of a flattened image.
For more sophisticated effects try Ctrl-M to bring up the Curves dialogue.
To change the shape of an object select it (Ctrl-A selects all) and choose Edit/Free Transform (Ctrl-T) or Edit/Transform and choose from the options. Drag the corner markers and press Enter to confirm or Escape to cancel.
To distort text you will have to merge it with an empty layer underneath. Add a new layer with Layer/New or click the new layer button in the Layers palette. Drag the new layer underneath the text layer. This text layer should be completely finished because merging it with another layer will remove the ability to edit it. Click on the upper (text) layer of the two and press Ctrl-E to merge them. You can now apply Edit/Transform to the text in the layer, though you will have to put a selection box around it first.
Check the more detailed lessons for further techniques.