When I am learning to ride SIF is it normal to not be able to ride very straight because of the tire wobbling back and forth?
practise, try keep the uni still. use search function 
yes your tire will wobble alot but with practice it will straighten out
sif is just somthing that comes with practice, when starting’ everyone is wobbly but with practice it will straiten out, ryan atkins rode sif along the back rest of a bench.
how to do start out doing SIF?
I just wrote about this a couple days back in a different thread. I’ll just copy and paste what I wrote there, here.
For seat-in-front practice, just ride around with the seat out in front of you.
Try not to let the seat push against your legs, but just brush up against your legs. Relax your arm that’s holding onto your seat. Put only as much pressure as you need to hold onto it. Use your other arm, that’s not holding the seat, for balance.
As you get better, trying riding seat-in-front up hills, down hills, backwards, turning both directions. You can also try to ride forward, and see how far you can hold the seat out in front of you. The more you practice, and comfortable you get with seat-out riding, you’ll be able to hold the seat really far out in front of you while riding.
I’m so comfortable riding seat-out, that it sometimes feels weird riding seat-in. As a matter of fact, I rarely do anything trials seat-in, anymore.
I suggest you get comfortable sif riding, before you start sif hopping, which is called jumping. But, practice hopping seat in, in the meantime, since it involves similar techniques and principles.
what do you mean? how to learn or how to mount?
never mind, trialsuni already exsplained it
the wobble is coming because you are not used to applying even pressure on the pedals. With practise it is possible to reduce this to pretty much zero.