Sivabiot: Your tank is too small for even betta , let alone a betta and two tiger barbs. Your tank is badly overstocked and your fish will not last long in it.
Also, tiger barbs are well known for being agressive when kept in ones and twos. They are notorious for picking at other fish when kept this way. The need to be kept in groups of at least 6 to cut down on agression towards other fish. These fish need 15-20 gallons minimum.
They may seem ok now, but in such a tiny tank and with nowhere to get away from eachother, you're heading for disaster with those fish. I don't know where you read that it was ok to keep fish in such a way, but the information was severely misguided/wrong.
Also, giving advice for hiding spots "in case things get messy " is really irresponsible. Things shouldn't have to "get messy" if you do your research correctly to begin with. Fast swimming species will stress the betta, and species such as tetras and danios for example, are well known for being fin nippers, also a bad choice for bettas with their long tails. Telling someone to keep tiger barbs?? of all things with a betta?, is pure lunacy.
Alannah In a 5 gallon tank ( good choice for a starter! ) your best bet for safe, low waste poducing tankmates is shrimp. You will need a mature tank ( at least 3 months ) so read up in the beginners section of the website, on how to do fishless cycling, and about the nitrogen cycle. This is really important because these are the two things you need to know about most, to keep a healthy tank( nitroen cycle ) and prevent your fish from dying or becoming diseased when you first get it (fishless cycling ) .
Once your tank is mature, you could get 3 or 4 shrimp to go with your betta. Cherry shrimp, Amano shrimp and Ghost shrimp are all good choices . They will eat any bits of leftover food, but you will need to supplement them as there shouldn't be too much extra food on the bottom anyway ( never overfeed! ) . Algae tabs and sinking wafers are always a good choice for this. Just one or two every few days for the shrimp is fine.
On a 5 gallon tank you will need to do 50% water changes every single week, even with only one fish in the tank. You need to keep the water chemistry in good order to prevent disease from starting ( you wouldn't believe how easily they can come down with illnesses, often overnight in some cases ).
You'll need a heater and a gentle filter for the betta ( they DO need filters despite whatever popular myths may say). The best sort of filter for them is a sponge filter. They run off a simple air pump and you can buy cheap ones on Ebay. A 25 or 50 watt heater set to 28 degrees C will be just right for a betta.
Plenty of plants for the fish to hide on and rest in are also needed. If you are a beginner and don't fancy messing about with live plants, then get several bunches of soft silk fake plants. Never get the hard plastic variety, these usually have little sharp bits on them which can easily damage the bettas delicate fins. Silk is best, and if they start to look a bit irty or grubby, you can just take them out and wash them under a warm tap, and plonk them back in again. Easy.
If you want other FISH to put with the betta, then please try to get a ten gallon tank. That can easily fit on a desk, and you could have some small bottom dwellers like Pygmy cories, or some peaceful mid dwellers such as the previously mentioned Harlequin Rasboros. These fish also need a mature tank like shrimp do, as they can be a bit sensetive to water quality.