So let's say I received 2 transactions to my wallet and I want to make a transaction. I want to choose which one's output will be used to sign my transaction. How can I do that?
In the Monero CLI wallet you have a couple of options:
If you received the two transactions to different (unique) subaddresses you can use the
transfer command specifying the optional address
index parameter (see
help transfer for details).
Otherwise you can use the
sweep_single command to send a specific output (see
help sweep_single, and example usage in this answer).