I had a vertical henox t3 bike which I hadn't ridden in a long time I changed gear and the derailleur bent into the wheel spokes and the derailleur is now bent and doesn't work properly. The derailleur is a shimano tourney sis index but I don't know the model. Can anyone help?
The first part to note is how the derailleur is bent. Is the frame aluminum or steel? If the former, is it just the hangar bent? (steel bikes typically don't have a derailleur hanger)
If the cage is just bent, you can take it to your LBS and they have a little tool to put in so they can easily unbend it (or you can do this with an adjustable wrench at your own risk, though i don't recommend it for people without experience).
If its just a hangar, you can spend the 5 dollars or whatever on a new hangar and be done wtih it.
If you do have to replace the derailleur, just get another shimano derailleur like a new tourney (these go for about 10 dollars) or acera unit - the rear derailleurs are pretty much all the same and the cheapest one will do in your case it seems (tourney again). Or check your LBS - they're likely to have a cheap one in their parts bin that fits. Then you can follow sheldon brown's advice to set the high/low limits and do adjustments.
Since the derailleur went into the spokes, I'd also get the wheel checked out.
You can find the model of your rear derailleur by looking on the backside for a stamp with Shimano's part number. Something like "tx55". You might have to flip the bike over and use a flashlight.