Ok so when after you've picked which team you want to play as (i picked man u) you will see you only have £500,000 (or whatever currency u chose) transfer funds which isn't enough to buy anyone.
Now you can try and play games to build up funds OR you can go to your squad list and release a player instead as you will get a fee for that player depending on his worth ie i released the worst player in my squad and got £499k for him which added to my £500k giving me £999k to spend. After that go back to transfer list and just look for a player that you can afford, i bought one for £886k.
Hopefully this is easy to understand as i know i waffle on a bit lol.