I'd recommend no less than a 60 litre tank for your first one.
The reason being it is large enough to keep the water chemistry stable, however not too big that maintenance is a headache. Larger tanks are easier to keep water chemistry stable, however you mention you are on a limited budget, and as a beginner there is no point shelling out loads.
I'd not recommend Bettas as a first fish. Not because they are difficult, but because as a beginner you may want to try a couple of different types of fish together, whereas Bettas are better on their own (males HAVE to be on their own or they will fight to the death ), so you may find it frustrating. You could keep several female Bettas in the tank, but to source good females can be pricey and not all fish shops have them in.
Angelfish's fins grow large, so they need a tall tank to keep them. You may be able to find a 60L tall tank, but to be honest Angels can be sensitive and do better in larger tanks where you can have a nice group of them.
I'd recommend some Tetras as a first fish. Some Tetras to look at are the Black Phantom Tetra, the Black Skirt Tetra and the Rummynose Tetras. I particularly like the Phantoms, and they do best in a group of 6, as they can be quite shy otherwise.
Get a good filter for your tank, and weekly water changes of 20/30% should keep the water nice and clean. Please don't be tempted to 'short-cut' and do a fish-in cycle, as it really isn't fair on the fish. You can buy household ammonia in Homebase and it's really simple to do a fishless cycle, no fish get harmed, and plus you can add your full stock straight away with this method.
Something like a group of 6 small Tetras, and maybe a pair of Bolivian Rams or a pair of Apistogrammas. If you can stretch to a 90/100L tank, you could add some Corydoras or something for the bottom. They like to be in groups too, minimum 3 but ideally more.
Best thing to do is go into some fish shops, have a look around and take down names of fish you like. Come home, Google them or ask on here, and decide whether they are suitable.
Good luck!