Assuming you mean looseness of the arm on the spindel and not of the BB
If you really have to do it yourself you probably need to tighten the fixing bolt should be either an allen (8mm?) or a 14mm socket. don't tighten it down too far though, although your crank may already be ruined from riding it loose, cranks are made from aluminum and they sit on steel spindels which will deform the arms quite easily if mishandled
Read this before attempting any maintenence, also verify that your crank arm is moving independantly of the oposite arm (the loose one moves and the other does not) if they both move do not tighten the fixing bolts
http://www.sheldonbrown.com/tooltips/cotterless.html
This all assumes you have a cotterless crankset
edit: afterthought, just read the entire http://sheldonbrown.com/fixed.html article
"and its hard to catch speed on it (fixed gear)"
I don't really know what you mean but if your saying what I think your saying (can't go fast enough) you need to change your gear ratio.

Reading through your post again I'm actualy not sure what kind of looseness your talking about if theres a nice LBS around your area take it in and see if they will diagnose your problem for free, they might.