Welcome. Your English is quite good. Your other tools is important to respond to your question. There are almost always other ways to do any particular project. I don't know what kind of saw you have, but nothing will work unless you have the saw blade a perfect 90 degree cut. You could use a chop saw (cheapest way) but you must make certain the blade is 90 degrees to the base. You can check this with a good draftsman's triangle. Most all cheap carpenter triangles are NOT square and can be off by 1-3 degrees, which will make it impossible to glue them and keep the frame square.
For starters, do you have your router in a table, or are you using it freehand. If in a table you'll be safer. A table can be made very easily from a sheet of plywood and a fence made of several strips of very straight plywood or a very straight piece of wood with vises to hold it in place.
You will need to make your long and short door pieces with a groove on one edge, then roundover that edge. That will take a couple of bits, one a slot cutter, the other probably a 6mm (about 1/4 inch imperial) roundover bit with a bearing. Cut the roundover first on one side. Then cut the groove. You can cut the groove with a slot cutter, then widen it to fit the panel loosely by raising the router bit slightly until the plywood panel fits. Plywood is a good choice for panels since it doesn't expand or shrink very much. Finish the panel before
you assemble it.
Be extremely careful choosing your wood for the long (rails) and short (Stiles) pieces that make up the door's frame. It must be very straight and you'll need quite a bit of it. You will also want to buy at least one extra piece of the same material for test cuts and alignment purposes, and to test your finish on. Unless you are painting the door, you should try to match the color and grain as best you can.
Make all the same cuts at the same time. You will NEVER be able to get a perfect match on height settings if you reset the router height between cuts.
If I were you, I'd make a quick router table first. Lots of plans on the site for making one. Keep it very simple as noted.
You will also need some clamps wide and long enough to hold the door in place while the glue sets. And you will need to go to youtube to learn how to use a tape mesure and long clamp to make your door frame square. You can use pipe clamps, which are reasonably cheap and can be adjusted for length. Use slow setting glue!!!! Give yourself time to fiddle with the frame. Regular wood glue is solid as a rock in a couple of minutes.
By the way,
The joint between the rails and styles will be a tongue and groove arrangement. That means that in addition to the groove in each, you will have to have a little extra length stile (short piece) so you can use your groove cuttter to cut the tongue by removing the excess on both sides of the tongue edge. cut stiles the length you want, PLUS double the depth of the groove for the tongue. See picture.
Trial fit on short pieces when cutting the tongue. You can rough cut first, then use sand paper on a flat wood block to make the final fit. It should be snug, but NOT over-thick.
If you use the table, cut everything with the final face DOWN
If you try this freehand, buy more extra wood and work with the final face UP
. If doing this freehand, you want to lay your workpiece next to the extra piece so you can steady your router across both pieces. Clamp the pieces down before you start cutting the grooves and roundover freehand.
Here's a video on cutting this joint on a table. Your table will be simpler, but the method is the same. The still picture is at the bottom of this post. This got a little longer than I intended at first, but as we say, the devil is in the details.