If your bed has bedposts, like wooden/metal poles on each corner, you can take some strong but not rough material such as a thin sheet and tie one foot onto the bottom left/right, the other foot on the other side, and the hands likewise. For a gag...I dunno, whatever fits. Maybe a clean cloth or something? Or you can buy one from a sex shop, but cloth is cheaper and easier to find.
And whatever you do after you tie her up is your decision. Have fun and use a condom. Oh and by the way, if your bed doesn't have bedposts, then lift up the mattress and stuff the other end of the rope/sheet thing under it, it should hold. You can be the master but show her lots of affection also.
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The best way to tie up your girlfriend for the first time would be with a light rope and a simple knot. This will make it very easy for you both to abort if something does not work out in addition to having a safe word.