3 month old filly. What do I need to be doing?

We wer given a 3 month old filly last night while selling some ponies. She is completely halter broken and leads without a problem. She also jumped right into the trailer. I picked up her feet this morning and moved them around. She follows me like a filly would do with a momma, nuzzling up and so on. She was not weaned! So she is obviously freaked out and lonely. I want to gain her trust. We don't have great grounds for lunging. Our area is pretty uneven. I know you don't start them that young. But what should I do when she does get big enough? Any suggestions?