I would like to change oil by myself, but I don.

You gotto do just like all mechanics do. There's some engines we see over and over and we know just which wrench to grab. But sooner or later an unfamiliar car comes in and we just have to try different size wrenches until we find the right one. Sometimes the drain plugs get stripped or the heads get rounded off, and they have to be replaced. There are a lot of universal drain plugs that work fine but have different size heads.

Try some different box wrenches. I strongly advise against using an adjustable wrench or an open-end wrench. Six-point sockets are the best if you want to prevent rounding the head off.

Don't get carried away when you put the drain plug back in. If it has rubber molded under the head, those are only supposed to be tightened slightly more than finger-tight. Make them finger-tight first, then go just a little more with the wrench. Another 1/8 turn is way too much. The threads on those plugs strip out very easily. They're designed to do that to save the threads in the oil pan.

Many Mitsubishi engines use a 17mm wrench on their drain plugs. For those, check for the presence of a metal gasket under the head. Those are supposed to be changed at every oil change or every other oil change, I can't remember for sure. Most people don't do that. I have two late 1980s Grand Caravans with 3.0L Mitsubishi engines and I've never changed those gaskets yet. If you want to, use a razor blade or utility knife to work the gasket loose. You'll have to struggle a little to get it off. The new one will slide right on. You can get them at any Chrysler dealer's parts department, most likely at a Mitsubishi dealer, and possibly other auto parts stores.