Hello and welcome

Don't worry about asking lots of questions that's what the forums are for
Right
The back of your work will improve with practice and experience. Mine were absolutely awful when I started but over time they have improved dramatically, thought they're still not as neat as I would like. However, nobody will see the back of your work when its framed etc so I don't worry about it any more
How to start without knotting the thread:
there are a couple of different methods to use. My favourite is the loop method. This only works if you are stitching with 2 strands of thread. Rather than using 2 separate strands, take one strand, fold it in half and thread the two ends through your needle. then when you place your first stitch, make sure you take the needle through the loop of thread. tada no messy ends or knots
The other method is as Pampam said. You use this if you are using a blended thread (2 different colours used together), one strand of thread, or if you are using 2 strands of thread and don't want to use the loop method. leave a tail of thread on the back of the fabric when you place your first stitch. Different people leave different lengths. When you do your first few stitches, make sure they lie over the tail of thread, so when you look at it, it looks like when you finish off the thread and the end of the thread is held in place by the stitches.
Some people (and kits) recommend using a waste knot to start off the thread but I'm not sure how to do that, I've never been able to get my head around it so I use one of the above two methods.
With regards to needles, it really depends on what you feel comfortable with. I usually stitch on 14 or 16 count Aida, but I've also stitched on 25 count linen/even weave. I usually use either a size 24, 26, or 28 needle and I find that is fine. the 28 is the smallest needle I use, and I usually only use that for very fine fabrics. I stick to a size 24 or 26 most of the time
There are lots of 'sticky' threads that have lots of tips for things like this if you get stuck
Hope this helps!