I have no idea atall how to make anew army. I have 30 warriors of rohan and 12 riders of rohan. Also Thorins company.
What would be the best use of these and what could I buy to get an army to learn to play with?

Any help is useful thank you!

Hello! This sounds a very good army. Basically you could now go for:

*Eomer + horse
-12 rider of rohan, 1 throwing spear

*Grimbold of Grimslade
-12 warrior of rohan + 1 item each + upgrade to s4

*Eowyn, armor, shield
-11 warrior of rohan +1 item each + upgrade to s4

Overall: 38 units, 500 points, 7 might...

All riders will already have bows, and you can consider whether you want how many bows to infrantry. Nevertheless, this seems relatively powerful?

For this list you would basically need eomer + eowyn + grimbold... It wouldn't be that expensive though i did not check specifics...

Basically I'd second Salattu's suggestion. Maybe swap Eomer for Erkenbrand so you can get the horn and the Westfold Redshield F4 upgrade for the Riders. Potentially put Eowyn on a horse to get some mobility for those 2 Might points.

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Thanks for the advice, going to pick up Erkenbrand for the cav, and then grimbold and Eowyn.

Sorry but what do you mean that riders already have bows? Is that just how they come? I presume that counts towards the 33% bow limit. Do throwing spears count towards the 33%?

Riders of Rohan have a special rule: they are not counted towards the bow limits. That means that if you have 30 units, 20 of whom are Riders of Rohan, the bowlimit is calculated on the remaining 10 units. Basically, you can have all the RIders of Rohan you want and you only have to pay attention to the limit when fielding Rohan Outriders or Rohan Warriors with Bow.
So, in the example above where you have 20 Riders of Rohan out of 30 models, only 3 other models can be equipped with bow. (33% of 10)
The reason is that, since the bow on the riders is mandatory equipment, doing otherwise would have made playing an All-Cavalry Rohan impossible.

Throwing weapons do not count towards the limit.