did u get 5 or 6 Floppies with WORKBENCH 3.0 with your a1200 ? , if you have the DISK labelled "INSTALL" then you just insert that and install workbench 3.0 (or 3.1 if thats what u got) onto your hdd.
anyway i would recomend you to get 1. A hdd , 2. A Acceleartor (any... 020 with 4mb fast is infact abig Speedup (just adding fastmem to a trapdoor card) .. 3. is to add a cdrom , but if u add a cdrom then u will need a new psu also..
i would advice you to look on EBAY and on AMIBENCH.COM for people selling second hand gizmos like 020 or 030 cards for a1200 and maybe a 2'5 inch hdd etc..
goodluck...