Taking a largish cutting from a yard tree is an unlikely way to start a bonsai. Most J. chinensis cuttings that succeed are the size of a pencil and no larger. You CAN strike a larger branch -- maybe up to an inch -- but it is a very iffy proposal for a neophyte.
As usual with first-time posters, I don't know where you live, but If you do this, do it in early spring. Make certain that you take the cutting with VERY sharp clippers -- NO ragged edges. While the cutting is fresh, dip the cut end in some Rootone, then poke a suitable sized hole in a bed or small pot of coarse damp sand, then press the sand around the cutting firmly. Keep the sand DAMP -- NOT WET. Full sun is preferable and do NOT let it freeze or dry out completely.
DO NOT move, jiggle, tug or otherwise disturb the cutting for at least three months. If you can stand it 4-5 months is better. If it dies it will turn brown in that time, and if you have followed instructions it will live or not; there's nothing you can do to improve the situation.
For your first tree, it is far better and easier to search the local nursery scene and buy a small nursery juniper and start from that.