No problem
The game itself is free so in effect you only need to purchase game content. As MissM said, you could subscribe to LOTRO for $15 to be considered a "VIP", then all the content excluding the expansion packs are available to you. You can purchase the Moria and Mirkwood expansions using Turbine Points which you earn through ingame deeds or you can purchase Turbine Points using your credit card or PayPal. Being a "VIP" subscriber also earns you 500points per month.
The only expansion you cannot buy with Turbine points yet is the upcoming expansion "Rise of Isengard".
I would suggest you download the client and create a Free To Play account and try it out first. You can play for quite a while before encountering anything where you would need to buy anything. Then I would suggest just paying the $15 a month to play and you'll have access to everything.
Any other questions, please feel free to ask me. If you don't want to ask on the forums, send me a PM and I will help where I can