mattoboy, are you thinking of Flourite? That comes pretty highly recommended. Caribsea Ecocomplete for freshwater fish is also a good brand. Be aware though that using these can cause a huge ammonia spike that will kill anything alive in the tank, so after you put in the substrate and water, don't add anything else until you've had the ammonia testing at 0ppm for at least 3 days.
Once you're sure the substrate has settled, you can add the plants. I use Seachem Flourish myself, but you won't need to add much if your substrate contains plant nutrients (and the two I've listed do).
Once the plants are settled and the tank has cycled, go ahead and add the fish :)
I will say, a 2.5 gallon tank doesn't give you a lot of space to get creative with when it comes to plants, especially if you want your betta to have space to swim in. I have this tank planted and I love it: http://www.amazon.com/Tetra-Crescent...words=aquarium
If space is a concern on your desk, a lot of people love this tank too: http://www.amazon.com/Petco-Bookshel...kcase+aquarium
I agree with mattoboy on those plants, they're all easy beginner plants. Make sure you don't give them too much light though.