On aviation exist 2 terms:
TA: transition altitude, this is the altitude when climbing you switch from QNH altimeter to QNE altitimeter 29.92 and you start flying FL (Flight Levels FLXXX)
You set the TA on the PERF INIT page on the bottom right corner, in USA the TA is 18000ft, other countries/regions is variable given their airspace rules. The charts specify this.
TL: transition level, this is the FLXXX at which you switch back to altitudes when descending. Altimeter moving from QNE back to QNH
You set on the DES page.
I do recommend the tutorials. See at the bottom.
Press the LSK X (left soft key) of the first waypoint below the disco (this will copy the waypoint name to the scratchpad automatically), then press the LSK of the disco and this will shift the entire flight plan one step up and clear the disco.
To delete a waypoint press DEL key (DELETE will be shown on the scratchpad) then press the key of the waypoint you want to delete, that position on the fmc will be cleared. Re-type the waypoint name on the scratchpad and press the soft ley you cleared to insert.
note: if you missed to insert a waypoint, suppose you entered A then C then D and you missed to enter B inbetween A & C. Simply type B on the sratchpad, then pess the softkey of C. This will shift C and remaining flight plan D, etc. one step down and insert B inbetween A & C.
Tons of profesional tutorials for the different phases of flight you will be required to understand and know how it is done on the real thing.
Hope this helps.