Focus is a high voltage function (but not as high as the main color monitor
anode, which is 24 kV or so). Locate the 'focus' knob (which, if you
are careful, can be adjusted while the set is under power). Sometimes
the back of the monitor has to come off, to get to it. It's always labeled,
don't touch anything that ISN'T the adjustment knob.
If tweaking it works, put the case back on and enjoy.
With simple tools, you can discharge the HV, attach a grounding strap, and
work safely on a TV or color monitor. The CRT is irreplaceable, and it is
NOT the problem, so don't mess with it otherwise.
If focus controls don't help, it's usually just
the carbon composition resistors around that 'focus' control. It is very likely
that some resistors, values in the range of 100 k to several megohms,
have failed. They're cheap, under $1 worth of parts.
Alas, color monitors may also need screen adjustment, and convergence and pincushion
and suchlike, tweaked as they age. Unless you find an old guy who used
to do this work, it's not gonna happen (there's lots of non-obvious steps
in the process).