If you can go in April vs. March, you'll definitely be better off weather wise. However, you could totally get lucky in March.
Since you're pretty new to cracks by your own admission, start easy and then work your way up. Also, I'd stay away from rope stretchers until you have a better idea of what you can handle, that way you don't have to bail and leave gear. Like someone mentioned, the first pitch of the Jam Crack is 5.7 hand. If you hop on that and it feels iffy, you need some work. If it feels easy, then you can probably head over to Bishops Terrace, Harry Daley, etc. I think Knob Hill has some easier cracks.
I'd try to avoid TRing established climbs if you can help it, especially if others are there who want to lead it. Poor form. There are some shorter cracks you can TR over at Swan Slab. Highway Star (.10a/b) is easy to set up a TR on. Ultimately though TRing will help your crack technique, but most climbs there you're going to have to lead to do, so you'll have to learn how to hang out and place gear too.