well can i tell them i'll be staying with friends the entire time?
You need to be honest with them - if they ask who you're staying with, tell them you're visiting your girlfriend/fiance/partner. If you say friends but then after further questioning it comes out that you are actually staying with a girlfriend, that will make your case even worse and they can refuse you entry for using deception (i.e.deliberately omitting information/lying to make your case look better) and they could even ban you from entering the UK for 10 years... that is much more serious than if you are honest, but are refused entry.
what evidance do they mean when i have to show them firm travel plans, evidance of savings and i need a letter from my landlord?
You need a return ticket (this is a must... no return ticket is pretty much a refused entry right away, especially if you want to stay for 6 months and are visiting your UK partner), your bank statements and any savings account statements (to show that you have enough money), and evidence that you have accommodation to return to in the US (a rental agreement, maybe a letter from the landlord too). You may not be asked to show all of this evidence, but it's best to have it with you, so if they ask for it, you can produce it.
To be honest, your safest bet here would actually be to apply for a visa in advance (either fiance or marriage visit visa) because then you would know before you travel that you have been cleared to enter the UK, instead of flying all the way there and then possibly being refused entry at the airport and being put on the next plane back to the US (at your own expense)... then, you could also marry while you're there this time
. If, for example, you came to the UK for 6 months, then tried to get a marriage visit visa for another 6 months, the visa may be refused because visitors aren't supposed to spend more than 6 months out of 12 in the UK (this probably wouldn't be an issue with the fiance visa though).