You can't get the tram all the way into the city. It's just a little connector line between a lot of bedroom communities and ends at Wimbledon. What you would probably want to do if you were in Morden would be take the Northern Line, Charing Cross train (there are two on the Northern line, Charing Cross or Bank) and transfer to the Central line at Tottenham Court Road, which is the most direct way. If you were closer to Wimbledon, you'd probably want to take the train from there to Vauxhall and change onto the Victoria line to Oxford Circle. A week's pass costs £33.20, a month is about £128, a year's season ticket is about £1,300, which gets you unlimited travel on the tubes, buses, tram, Docklands Light rail and the commuter trains within Zones 1-4, I THINK Wimbledon is actually in Zone 3, could be wrong, but it would be a bit cheaper.
Since you're going to a job already, you want to get your employers to make you a season ticket loan so you can get the best deal, which is the annual pass. Then they usually just deduct 1/12th of the cost from your monthly pay.