Did you ever experiment with any steroid before? If so, how long ago?
I'd always suggest to try to start with gel at first and see how it goes, possibly later switch to injections but not as the first thing specially at your age. Trt is usually for life. Gel will still cause infertility as well as injection.
You have to ask for the Hcg and express you're worried about the baby issue.
It would be good idea to do sperm count BEFORE you start trt because it also means alot in this case, regarding your future and sperm count.
The half life of cyp is approx 8 days aka bit over week or so. It's individual for all, just an approximation.
After week you have 50% left, after 2 weeks you have 25% left and divide the remaining dose every 8 days by half. The stuff stays in you longer than you expect it does.
The injection in smaller dosages more frequently is to prevent peaking and levels from fluctuating so much. With injection i don't think every two weeks is the way to go with 100mg, none can tell how you will feel with it but levels will fluctuate and you could notice big highs and lows in energy levels ( not only in gym, in normal life ). Your bodys hormonal function certainly will.
Later on the fertility issues can be tried to treat with Clomid aswell and it seems to be the popular choice at the moment atleast. It's no where near 100% guarantee you will get your sperm count back to what it was. However the state you were before starting trt also matters in this case.
In the end, trt can cause the downside of permanent infertility specially if you had low sperm count in the first place and is to be considered if the trade off is worth it to you personally.
Talk with the doctor about this.
Somehow i see stories about very irresponsible doctors in this forum very frequently and it makes me wonder.
It's really great thing you think about this as not many guys in their 20's gives a flying shit about possible future kids.