That's a very broad question.

What kind of restraints do you want? Simple handcuffs, tethers to the bed posts/head-board and foot-board (if you have a bed with those features), or simple under the bed straps?

I personally have had decent luck with sportsheets products (though leather products are nicer).

For simplicity I love the shortsheets velcro sheets. I leave them under the normal sheets on the bed under our normal sheets, the velcro feature allows you to place the restraints anywhere you want. Plus as a beginner they restraints won't budge (I have curled the mattress with the velcro) but a twist of the hand and a grab of the base and you can break loose.