Top Spin 4

Select 'Xbox Live' from the main menu, then 'World Tour'. This is where all the tournaments and other gubbins are located.
All you have to do is select 'Quick match' and you will enter the lobby for that event. Once the countdown reaches zero (or sooner in some cases, for example if the lobby is full), you will be matched up with an opponent - if no-one else enters the lobby, then a new one will begin.
All you have to do is win the match, but that's easier said than done, as most people online are quite awesome...it is only one set, so is easier than the tournaments.
As for actual tactics, most people seem to have massive serves and high power ratings, and just hit the ball to either corner to make you run around quite a bit, so if you have good stamina, that's a plus. Also, good drop shots and watching their movement helps, as most people hit from one side then move to the other, so if you hit it to the same side they came from with a power shot, that will wrong-foot them and they will be unable to get a shot off.

If you are boosting this with someone, it will help to both have female characters, as this increases your chances of being matched - i would go to the lobby as a female on my own and always be matched against a male player.
Good luck!