Cannot start SQL Server replication as already running
You can do this via several methods. (from memory as don't have a replication environment in front of me!)

1- My Preferred intial method -  Open the Replication Monitor. You can do this by right clicking the 'Replication' folder in Management Studio and selecting Replication Monitor. 

You will then have to register your publisher if this is the first time and add your Server into the list with the correct credentials (This can be local to the Publisher or can be setup on a remote Distribution server). 

When you have selected your publisher you should now be able to select go to the agents tab on the right hand side. You can then right click and select 'view details' This should hopefully give you further insight into why the current replication is not working and its status. You can also stop and start agents from the monitor.

2-  The other method is to go to the SQL agents job on the server that is running your distribution DB and open the SQL Agent. you will see several jobs relating to replication.

Expand the jobs that are marked failing and look at the job steps (you will often have to go to the step before the retry to discover the real reason why the agent is failing). Obviously you can stop and start any agents here, but nearly always you will experience the same result. you should try to remedy the underlying issue first.  This will all depend upon what step is failing but permissions is a fair bet with Replication.