If your going to Disneyland I heard it's good to go early on Sundays, Wed and Friday's as there are very few lines. Do research for the rides with fastpass, and don't go on them in the morning, go to the big rides that have long lines normally (and don't have fast pass) first, since you can skip the line once for the ones that do.

Supposedly that is still the time (opening, which I think is 8 am) when the least number of people show up.
Also if your not going with kids etc, you probably can actually walk faster then many of them and get
to the rides first.

Plan out your rides, grabbing the ones with longer lines etc right away, then hit the fast pass ones and you
won't have to wait in almost any lines (they can be over an hour sometimes otherwise).

The cost for a ticket varies, but it's about $100 or so ($120 normal I think, $100 value). Fast pass is free
but there is something you can pay for to let you access them on your phone, so you can schedule
them before actually getting to the ride (which can literally mean you jump right on the ride as soon
as you get there). The cost for that is an additional $15 per ticket.

Avoid going near holidays as the lines get much larger during that time.

Supposedly none of the lines are over 15 minutes (when they can be over an hour normally sometimes), early in the morning on the above days (I'm sure there are exceptions). That makes fastpass less important, but it's still nice to have. One.....interesting thing is you get 1 fastpass a day....but if the lines
are short, you could get a fast pass, go on the ride, get off, and go right back on it (with no wait), since
you have not used your fast pass yet. So if there is a 15 minute line, and you really like a specific
ride, you can go on it twice (main reason for this is fast passes are set for a specific time frame. If
you get to the ride 20 minutes before the time frame starts (you only can choose between options), you
could stand in line for 10-15 minutes, do the ride, and then be within your time frame to do it again
without waiting.

Anyway it's not THAT expensive over all, considering almost all parks cost 50$ or so at this point.
I'd aim for a day that isn't to hot/sunny if you can, going early means you might be done by noon
even, or at least ready to take a break in the shade by then. California can get pretty hot though, this
is actually a pretty good time to go if the entire state wasn't on fire again.