This is a really late post but others might find it useful ...
there's a rule of thumb you can use that generally works OK.
ASA film speed is a rough meter of shutter speed, i.e.
ASA100 will shoot well at 1/125 of a sec, ASA500 at 1/500 sec etc, such that:
Dim light, overcast day - use f4 set on the lens
Cloudy day, typical spring morning, white clouds - f5.6
Bright day, sparse clouds - f8
Summers day - f11 to f16
As an alternative to 1/125 at f4 (say) you could use either 1/60 at f5.6 to get more depth of field, or 1/500 at f2.8 to isolate the subject better or to freeze motion.
The general rule is that if you slow the shutter speed by one incriment (stop) you need to use an aperture smaller in size (bigger in number) by one stop and vice versa.
The generally recognised "stop" shutter speeds are 1, 1/2, 1/4, 1/8, 1/16, 1/30, 1/60, 1/125, 1/250 and 1/500 and these are likely what you have on your Zenit - I have half a memory of there being a 1/1000 on the later ones ...
The lens stops that correspond are: f2, 2.8, 4, 5.6, 8, 11, 16, 22
Thse rules served me well when I was a lad, equipped with a very second-hand Zenit 3M (which I still have and on occasion use, for old times' sake) & no light meter, back in the 1970s!
Indoor exposures though - much harder to judge. Flash exposures ... be careful as some Zenit cameras synchronise at different shutter speeds - M synch types synch at 1/30th, I think, X synch at 1/60th if my memory is to be trusted ....
BTW, if you are really new to this, and you want to do the whole retro thing with black and white film, you might also want to look at a red filter, 1/2 to 1 stop strength (i.e. you have to slow your exposure down by that amount) - B & W film renders blue as white (mostly) so those white fluffy clouds in your landscapes will disappear without it :-(
