For close quarters you should use an SMG. The MP7 is a little weaker than the MW3 version but it's still pretty good. However, one of the default classes has the Skorpion and once you get used to it, it's a really good SMG. For longer range fighting and just starting out, I'd go with the MTAR. Its pretty accurate and plays like you would think that an AR would. As you level up in Multiplayer you will find a number of other weapons that perform very well. I suggest just trying them all and seeing which ones fit your playstyle the best. Some guns have a learning curve to them but once you've figured them out there really isn't a bad gun in the bunch.