First off, welcome to Tropical Fish Keeping forum. Nice to have you with us.
Filtration should be based upon the needs of the fish that you intend to house in the tank. Not all fish need the same amount of filtration and associated water movement, so this is important. Also, if live plants are intended, that makes a bit of a difference.
Generally: An internal filter such as one in the Fluval series by Hagen is one option. A sponge filter would work if you intend live plants and forest-type fish. A canister would be better if more water movement is needed, as for larger fish (though not too large in a 35g) or fish coming from fast-flowing streams.