3 month old sleep help!

My baby will be 3 months soon and until now I have to hold her so she can fall asleep, day and night. I think she sleeps pretty good at night I'm able to put her down 20 minutes after she's fallen asleep and she will stay asleep. The problem is during the day... she will wake up immediately once I set her down and the only way for her to nap a good amount is if she stays in my arms. My ultimate goal here is for her to fall asleep by herself Day and night. Especially coz I'm going back to work soon ... how do I go about winning in this situation??? Thanks!