Munich itenrary

Salzburg is an easy trip, just 1 hour and 45 minutes on the regional Meridian train. You can use a Guten Tag Ticket for a round trip from Munich if you leave after 9 am on week days. This ticket is good for the entire day between Munich and Salzburg and costs 21 EUR for 1 person, so covers the trip to and back. These trains go every hour (almost) and leave from Munich Hbf at 5 before the hour. To get an earlier start, you can take the 8:55 train by buying a local transport (MVV) ticket for 2.80 EUR and travel to Munich Ost station on the S-bahn and board there. The 8:55 train stops there at 9:04, so then the Guten Tag Ticket is legal. I am not sure if MVV tickets are valid on Meridian trains, they might be, but to be sure, take one of the very frequent S-bahns.

If you get snow in the mountains, also look at a train to Mittenwald to experience snow in the Alps. You can use a Werdenfels Ticket for 20 EUR, again valid for both to and back and leave after 9 am. It is 1 hour and 51 minutes from Munich Hbf.

Nuremberg is also a good day trip and you can use a Muenchen-Nuernberg Ticket for 20 EUR, or with a Bayern Ticket for 25 EUR. The advantage of the Bayern Ticket is that it will also include all public transport in Nuremberg (and Munich as well). Same deal, travel after 9 am and be sure to use only regional trains. It will take about 1 hour and 50 minutes. Also be very careful which regional trains to use as some can be very slow, making every stop and taking 3 hours or more. Also there are fast ICE trains that operate this route. They are a lot more expensive and require separate tickets, so unless you have money to burn, don't use these.

Vienna, Prague, and Budapest are all too far away for a day trip.

Other doable day trips are Regensburg and Bamberg. Both are within Bavaria and so within reach of the Bayern-Ticket.
Use the website of the Deutsche Bahn for travelling times.

I am not sure if MVV tickets are valid on Meridian trains, they might be

Legally, inside the MVV there is no difference between a S-Bahn or a RE, Meridian, BOB etc.

Good to know. That will save a bit of bother when taking the 8:55 with a GT or Bayern ticket.

If you look up Munich Hbf to Munich Ostbhf on the Bahn website, it doesn't show a fare on the Meridian train; it just says it is an MVV fare. Same applies to any regional train entirely within the MVV.

Per the MVV website, if you take the S-Bahn from the Hbf to Ost, it's a one zone ticket for 2,80€, but if you take the Meridian train from the Hbf to Ost, it's a Kurzstrecke (short ride) ticket for only 1,40€ (and you don't have to change at Ost).
The only train that's outside the MVV ticketing would be an EC, and the Bayern-Ticket wouldn't be good on it anyway.

I would read the reviews on Trip Advisor about the hoho bus tours. They aren't very good. Better would be to dress warmly and join one of the many walking tours of Munich. This gives you the best orientation to the city as well as all of its' history. You can ask the guide questions and perhaps find new places to eat, museum recommendations, etc. For 14€, which is what most of them charge for a 4 hour tour, you can't beat the price. I would avoid the "free" tours.

11th to the 15 th of december so i need ur recommendations where to go there or what to do

Are you arrive the 11th and departing the 15th? If so that give you three full days in which I would recommend visiting the various Christmas markets around town, Deutsches Museum, the Residenz and Nymphenburg.

also i want your reviews about the munichen hop on and hop off

Barely worth it in summer when the weather is nice, definitely not worth it in winter. You are better off getting an inner ring day pass and seeing the sites on your own by public transport (the number 19 tram between Ostbahnhof and Hauptbahnhof give you a nice ride through the city for starters).

Then i m thinking of visiting another country after by train and im confused between salzburg or vienna
,or prague or budapest

Salzburg is a easy day trip from Munich and quite nice in December.

DJ