hmm a 17 and a 18? Not bad not bad.
You could do 18 Strength 17 con, and fill out the rest however you want. If you absolutely want 20 strength you can go human and put your racial points in there. Though you might want to consider putting one of the 13s into Dex and putting the 2 into there for 15.
Power Attack is always popular. Do you plan to be using your horse in combat a lot?
Basically you have one main stat, Strength. It will determine your hit chance and your damage.
18 is great, 20 is phenomenal.
Then Con and Dex will keep you alive, Con adds straight your fort saves and HP, which is great. 14 con is good, 16 is great. Dex adds to your AC and your touch AC etc. However the heavier your armor the less of your Dex you will be able to use, so 14 or so would be fine. Those are your main stats. Your mental scores don't do anything for your Samurai mechanically, so pick them for how you want your character to be. For race, pick anything. Whatever you want, you have 18 and 17 with out racial mods for Strength and Con, so pick something based on flavor and what you want.
This assumes you are going to be a classic sword fighting melee samurai.
I haven't played a Samurai myself. But all your full BAB melee classes are built similarly at this level. But hopefully some others will pipe in with some more experience and tips.
Sarenrae is Neutral Good and shes big on smiting evil doers, but helping them redeem if they want to. Sun goddess and all that, uses a curved sword (Scimitar but I digress) Shelyn is also Neutral Good, Goddess of beauty and art and all that.