Wars you're engaged in will show up in the Our Wars tab (under the Wars tab, under the Corporation tab in your Neocom), as will any wars that you're an ally in. If you're a CEO or Director, you can list yourself as Open For Allies and decide whether or not to accept offers for help -- which you should expect, naturally, to come at a price. Negotiation of prices can be carried out through normal communication channels, as the current UI allows you only to accept or decline the offer.
If trouble hasn't found you and you want to spice things up, the All Wars tab is the place for you: It's a list of, well, all active wars, with information like who's asking for an ally and wars with one or more allies. By default, the list shows the 50 most recent wars, but it's also searchable by corporation/alliance. There's an "Assitance requests only" box that you can check to speed up the sword-selling process. Once you've made an offer, possibly done some negotiating, and been accepted, you'll be counted as an ally, although you'll have to wait 24 hours to start fighting.
The purpose of this system is to give more options to defenders, introduce some risk for attackers, and provide ways for mercenaries to support themselves. There's a whole list of extra rules and explanation over on the blog proper, so be sure to check it out.