It just takes a little practice. Once your fingers get used to it you'll discover that it is one of the easier chords - and it's also a closed chord which means you can use it all up and down the neck.
That said, depending on how limber your fingers are and what size uke you are playing there are a couple of things that might help you get there.
First, finger an F7 with you index finger on the 1st fret of the 2nd string and tip of your second finger on the 2nd fret of the fourth string (the standard F and you already know it). (You should always use the tips of your fingers, not the pads, but in this case it's crucial.)
Now, to get the Dm fold your finger as if you were trying to bend it backwards - the pad of the finger should fold right down against the fretboard and now that finger is barring the 4th and 3rd strings at the second fret. That's the Dm - easy peasy. Now, to get the Dm7 just drop your pinky onto the third fret of the 1st string.
You probably haven't used your pinky much up 'til now, so it may be a little tender until it toughens up. Once it does you'll wonder why you ever thought the chord was hard...really! I promise!
BTW, this making the Dm by folding down the second finger really pays off if you want to play blues shuffles. The reason is, a Dm is also an F6 so now you can "dance" through the F F6 F7 F6 sequence really easily.
See, the cool thing about learning an instrument, especially a stringed instrument, and especially the uke, is that every time you master one thing you are actually mastering several things!
Finally, while you're young enough that I doubt you will have any trouble folding that finger "backwards" to make the barre - if it is too difficult for you then you can actually use your ring finger on the 2nd fret of the third string. The reason I don't recommend it as a "first resort" is because it will slow down things like blues shuffles.
John
OMG I can not feel the tips of my left fingers, my iPhone doesn't even respond the same when I play games, hah! They don't hurt anymore tho. Need to build up my pinky!
