If you have been challenged to a friendly match...
In the grey bar near the top, click 'Club', then in the green bar just below the grey, click 'Challenges' (it's the 2nd from the right. looks like a magnifying glass over two ovals). From there, you'll see all potential challengers. Just click accept for the one you want.
Note: if you have a message that says you've been challenged, but there are no challengers showing, that means they challenged multiple teams and another team has already accepted their challenge.
To Challenge another team, click on the team page. In the area where the stadium is (or should be), there is a 'Challenge' button in the upper right corner. Click the button, then use the drop down to pick the stadium, (yours or the challenged). If the 'Challenge' button is greyed out, then that team already has a friendly scheduled for the upcoming Monday. You can only schedule one friendly per week.
If possible, try to find a team with a stadium with 13500 seats or close to it and play the game at that stadium. If they have built their stadium correctly, both teams should bring in $150,000 to $200,000 each for the game. When you start getting into the big stadiums, the ticket money gets diluted with all the regular tickets prices. For friendlies, a big stadium doesn't mean big attendance. Expect attendance of not much more than 15,000 at the best. Usually, it's less.