Hello Erratic.
When you log into steam, It pops up a window saying press this nice button to setup your steam id. if you did that then you steam id is the name you show up as on friends and in game. If you missed that, right click on the steam icon and choose friends. then yours should be at the top of the list with a link to open user control panel. do that and then click on the blue edit steam id link at the top. you will then have a profile page open.
On that there is your current steam id. change it to what you want e.g (=XDC=Erratic-Space) choose your avatar (this appears on friends and in game score board) and some info then click apply and your done.
ppl can add you as a friend by your steam sign up email address or your steam i.