Kyle Field is largely sold out of ST allotments but there are some available. For a new STH any time is the best time, as you are new and at the end of the line anyway. As TxAggieBand85 said, go ahead and get them now and choose to opt-in for seat readjustment period.
Renewal is in Jan. - Feb, some time in April the season ticket holders have a chance to upgrade / revise their seat selections based on availability. This is done in order based on priority points.
If you are getting just a pair there is much more opportunity to move seats with an increased donation amount. if you have 4 or more it is more difficult.
You should probably call the TMF and ask if you become a new STH now will you get to participate in the seat readjustment period in April, my guess is yes.
There is always some availability as some don't renew, drop tickets, move seats, etc. No matter your priority point level, once you have seats, they are yours so long as you renew them. So, you could get a pair in the upper north EZ, but during the seating adjustment period you could move to 2nd deck west side 40 yard line if a pair was available and you wanted to pay the extra $.
Hope that all helps.