Hecarim on top isn't that good...I tried him a few times on top, but he's better in jungle.
When it comes about the items, just grab
and some tank items, it depends on the enemy team:
If the enemy has some strong mages, then pick 
If the enemy has a fed adc or a strong ad mid just get
.
Also, you can take

or even
if you're fed. This new item is good on Hecarim.
Usually you will go for a build tank, since you're the frontline...your role is to disrupt the enemies during a teamfight, so building armor, health and mr will be great. If you get some early kills go for
. If you fall behind just get
. You'll still be useful.
An usual build will look like this:


You can make a lot of builds on Hecarim...I prefer to get
when I am ahead, especially because his damage scales with movement speed.
You should choose the clear path...usually I go gromp->blue->buff->gank->recall then go for the other side of jungle. Also, don't forget to take the scutler. Is important because it provides you vision and some movement speed when you're in that area.
In teamfight, just go with E-R but be sure that your team can follow you. Max Q-E-W.
For runes: Quint of ad, marks of ad, seal of armor, glyph of magic resist.
For masteries you can go with 18-12-0 if you wanna be very offensive. 6-18-6...or even 6-6-18 if you wanna be more tanky. Hecarim is so versatile, he can adapt to almost every playstyle.
Good luck!