Moving around
←↑→↓ (arrow keys) to move around. [E]/[C] to fly up/down. [F] to stop flying. [Ctrl]-[R] to run.Camera
[Alt]-click to zoom, [Ctrl]-[Alt]-click to rotate, [Shift]-[Ctrl]-[Alt]-click to pan.
[Esc] to return the camera to behind your avatar. [M] to view from the avatar itself (movement controls still work, use the mouse to look around).
[Ctrl]-[Alt]-[D] to turn on the Advanced menu; enable 'Disable Camera Constraints' (allows the camera to move independently of the avatar), disable 'Limit Select Distance' (allows selecting far away objects), enable 'Mouse Smoothing'.
Display
Edit > Preferences > Graphics.
Move the 'Quality and Performance' slider to 'Ultra', then tick 'Custom' to show the advanced settings.
Disable 'Basic Shaders' (the only setting that makes a big difference to the frame rate) and increase 'Draw Distance' to the maximum.
Disable 'Advanced > Rendering > Types > Clouds' and 'Advanced > Rendering > Features > Fog'.
In 'Advanced > Debug Settings', set 'RenderDynamicLOD' and 'RenderUseFarClip' to 'FALSE', so that objects in the distance are rendered sharply.
In 'Edit > Preferences > Network', set 'Maximum Bandwidth' to the maximum.
Snapshots
In the Advanced menu, enable 'High-res Snapshot'. When you take a snapshot, switch 'Basic Shaders' back on in the Graphics Preferences, and enable 'Anisotropic Filtering' and 'Antialiasing' in 'Hardware Options'. Change the output format to PNG, set 'Size' to 'Custom' and set the width and height as high as they'll go. See Torley Linden's Guide to High-Quality Photography for more.
Purchasing
You can buy objects directly in Second Life, but it's easier through XstreetSL, which has descriptions and ratings. Whichever you choose, buying currency involves two steps: transferring US dollars into your account using PayPal, for example, then using that to buy Linden dollars on the currency exchange. Once you have L$, these can be used to purchase objects.
Once packaged objects are in your inventory, drag them onto a patch of land to rez them, right-click the rezzed object and choose 'Open', then choose 'Copy To Inventory'. Now you can make use of the objects.