First you need to feel you can trust your partner.
Second learn to control the kegel muscles, once you can relax and tighten them you will be able to play with large dildo.
Third Forplay is a must the hot and horny you are the better.
Fourth LUBE my preference is sweet almond oil. You can get it at most health food stores.
And Lastly we all have our limits and if we push it to far we can end up in pain.