There's tons of different audio interfaces. Depends on your budget, also think about how many inputs you need to have. Also, there are audio interfaces with Midi In/Out and without. Think about it if you'd need one (for instance to send midi notes to AE Modular). And also there are audio mixers with USB function, i.e. you can connect mixer directly to your pc and use it as an external soundcard. But the difference usually between audio intrface and the USB mixer is that with audio interfaces you'll get more than one audio input function. And audio mixers usually have a lot of inputs, but only one goes out (to your DAW, already mixed)
You will need a USB Audiointerface to get audio signals into your PC. There are many with different price points, here is a small selection:
These guys are all very similar in that they have 2 audio inputs and two MIDI ports. The MIDI ports are very handy for sending MIDI signals from your PC to the AE Modular. I've listed them in order of price with the first being the least expensive.
Steinberg UR22mkII USB
Focusrite Scarlett 2i4 USB
Alternatively you can also get an audio mixer which includes an audio interface. These can be a bit cheaper, and also you can get them with many more audio inputs, but they will all be mixed together and sent as two channels to the PC. Look at:
Behringer Xenyx Q802USB - which has 8 audio inputs which you can mix together into 2 channels to PC
Behringer Xenyx 302USB - has one microphone input and left/right line level inputs.
Yamaha AG03 - does pretty much the same as above but is much more expensive
You must also take into account that all the cables that you connect to the audio interface are either RCA type or 1/4" mono connectors. AE Modular however has only 3.5mm jacks, so you will need to buy some connectors.
Regarding to careck I could add that Behringer (at least if you choose a budget version) might have some noise issues, I had some time ago a mixer with 8 input channels, now have a super simple smaller version Xenyx Q502USB and it might make some noise. So you should consider if you're looking for a quality stuff - you might need to invest a bit more. Other than that, for the starters - that's a way to go. After awhile you might just sell it if you'll feel like you need an upgrade. Never tried Zoom but my guess that this might be also a good budget/beginner option. Altho, Behringer audio interfaces, newer versions (and not all of them, but you should check for specs) have a Midas preamps, which is qualified as a great quality audio tech.
I’m really a big fan of Zoom. It’s all plastic but it sounds alright and you get lots of features affordably. Had my Zoom R16 for years now. It’s a stand-alone 8-input multitrack recorder, which can act as a USB audio interface too.
Other than that, I also use the Zoom U-24 with my iPad, which is a portable ISB Audio Interface with MIDI I/O. It costs less than 100€! There is also a version without MIDI and one with digital I/O (U-44).