It's a deep subject. My answer is to make a list of all the features you need now and possibly for the future; A simple interface like the Scarlett 2i2 is an example of a good starting point at $125 or so. But for me it didn't have enough features so I went with the 6i6.
List of stuff I look for.
How many mike pre's
How many overall inputs, line level, Instrument, SPDIF and MIDI
How many outputs, Line level, SPDIF and MIDI
How many headphone jacks and do they each have a level control
Front panel control of input, pad, headphones and most important Monitor level.
Are the inputs combi jacks or is there only one xlr and one instrument
Is it Bus powered or does it use a proper power supply
Is there a USB hub for your controller ( new feature on some which is handy for laptops)
Is the case metal of cheap plastic.
Does it come with a software GUI mixer and/or DSP effects.
Does it come with useful free software like an LE version of a DAW you would like to try out or some cool VST plug ins. .
And most important of all - Does it come with the correct drivers for your system and there are no complaints from the rabble on the audio forums.
If you want to use real time efx for guitar sims watch out for pour round trip latency performance.
They all claim low latency monitoring which is something different.