Crib Tips

My baby is 4months old and has co-slept with me and my fiance since the first week he was home. Now don't judge, desperate times calls for desperate measures. But I can no longer co sleep with him anymore lol Me and fiance are just to crammed together, which isn't comfortable anymore. So how do I go about putting him to sleep in his crib? I have tried putting him in there several times, and he only stays calm for about 15 minutes then starts crying. I need help!