Both of these two sounds is actually a combination of two different sounds. The "ch" sound like in "chip" is a combination of /t/ as in "tip" and "sh" like in "ship". The "j" sound in "jet" is a combination of /td/ as in "dip" and the sound in the middle of leisure and measure.
They are both made the same way in your mouth. The difference is that "ch" is made without using your voice, and "j" is made by using your voice.
To make these sounds, put your tongue on the roof of your mouth, right behind your teeth, then move your tongue back behind your teeth until you are making the "sh" or "measure" sound.
If you are having trouble making these sounds, try these tricks. To say "chip", practice saying "meet ship". Say it faster and faster, and then you will notice that it sounds like you are saying "me chip". If you are having problems with "jet", first, say the word "measure" several times. Now try to add a "d" in the middle of the word, so you are saying "meadsure". Try saying this fast. It will sounds as if you are saying "medjure". Now you've got it!
Watch videos for these sounds! In these video clips, notice how the speaker's mouth moves in a very similar way for both 'church' and 'judge' -- the difference in how she says them is with her voice. (These videos will launch a separate video viewer to play them. Make sure you have a plug-in for video clips.)
Now let's listen to some sounds!