When the Duel starts, you will be shown a brief screen of you and your rival facing off, then the dueling screen, and then your Card File will immediately pop up, filling the left half of the screen.
Why not start with a level 2 card? You put 2 SP into your Sphere of Influence, 2 into summoning the card, and you have 1 left over. You'll also want to choose a Unit or a Character type card. There are also Grimoires or spell cards which have instantaneous effects, but you'll want to wait on those until you have some Units on the field.
To play level 4 Refess (yellow) you'll need 4 SP and level 4 in your Refess Sphere of Influence. In other words, card level = SP cost = required Sphere Level. You'd need 8 SP to play a level 4 card first turn, and you only start with 5.