Look through the models and just decide what you think looks cool I think is the best bet. Its good to try and start off from a starter set though since its cheaper. In terms of starting off with a sectorial, its fine. Its what I did and you just get used to it cause its what you have. I went with corregidor nomads, adding a few wildcats (medium line troops) and a couple of hellcats (but corregidor is a bit more straightforward). Took me about a month to choose what exactly I wanted though (kinda went through every army). The main things I looked at were; the models looks, how it played and the army playstyle it'd work with and its background fluff.
So I'd just flick through their site and have a spin on the army generator and see what appeals. Can also get links to the rules from it. I looked at
CA a bit, ones that appealed to me in particular were the anathematics, I mean a massive
TO tag with sepsitor has a bit going for it. But really you can go with whatever you want and it should be fine. There doesn't seem to be that huge a gap between a bad army list and a good one. Just dont spam the same thing like in
40k, a bunch of combi rifles are always handy but wont do anything on their own. You want a bit of diversity in your loadouts.
Now I dont profess to be a good player; still a bit noob as they say. But seeing as I have a couple of options for what I could field at a 250pts level these are some of the things I try to include:
- Lieutenant; either a basic guy so you have a couple of them (e.g wildcats) and then your opponent has to guess which one it is. Or a beast guy, obvious but harder to take out (I've never gone this route so...

)
- A few cheap guys to help with getting orders and not retreating, works nicely with the obscured lieutenant
- Try to use up the majority of the
SWC allowance. I normally look for a couple of
EXP things to deal with hard stuff, so thats DEPs and a rocket launcher for me.
- Something to give a bit of flexibility and surprise for your opponent. For me this was the hellcats with
AD: Combat jump, an intruder with Camo and a tomcat with Infiltrate. Its necessary to have something that can deal with snipers as I found out in earlier games against a high vantage hassassin and a lot of
AROs. The hellcats and tomcat allows me to combat these guys quickly, or work on other opportunities. Whilel the intruders camo is useful, I really need to get another with a
HMG to get the most out of them. Unsurprisingly the shasvasti have a lot of options here, I'd just be concerned about things with total reaction if I was them (which aren't many).