Hello everyone, I recently downloaded Krita. Full disclosure: I really have no idea what I'm doing.

Long story short, I recently got my self produced music placed on iTunes, Spotify, all the music services, and I now need to start designing various merch items. I'm trying to make a design of the band name to be used on shirts and whatnot. I have a font picked out for it, but as far as the color goes.. Its a three word name (not sure if advertising/soliciting is allowed here, so I'm just being vague lol). The first two words will be a light blue color, and for the third word I'd like the "color" to actually be an image of a brightly colored galaxy. I'm not exactly sure how to explain it, like you might see a word colored to look like zebra print? Well, I'd like this to be colored to look like a galaxy. I'm not sure if I'll need a picture of an actual galaxy in order to do this, but if so I have a few I can pick from.

I'm pretty sure Krita can do this, but i have literally zero idea how to make it happen. Truth be told, I'm not even sure how to create a text field yet, or even what my measurements should be when I first create the project, but I found some tutorials here that will help with that. I appreciate any help you can give me. Thanks!

Hello and welcome!
For more information on any of the things I mention below, please refer to the User Manual.

In general the image size should be pretty high for a high quality print, and the dimensions ultimately depend on what it ends up on. A t-shirt print won't have the high fidelity of, say, a CD cover - and may require an adjusted design. The printing company of your choice might have some suggestions listed. I'd go with 4000 pixels or more on the longer axis, with 300 ppi or more. The default sRGB profile should be fine, and if you want to guarantee super smooth gradients you can set the depth to 16-bit integer/channel. Downgrading the quality is always possible afterwards.

Note that the color and brightness gamut in prints is somewhat limited, if that is critical to you have a look at Soft Proofing in the manual.

Now for the design itself:
Creating text is done with the Text Tool, which is the second item in the Toolbox. Click and drag on the canvas and a pop-up will open that lets you input your text and edit font, size, color, etc. for every letter selected. Click Save to update the text on canvas and then Close once you're done. Optionally you could make a separate text object for each word. Text is automatically placed inside a Vector Layer, just so you know.

To edit the text again double left-click on it with the Text Tool or the Select Shapes Tool, which is the first one in the Toolbox. The latter also lets you reposition the text object or give it an outline or fill from its Tool Options. Make sure you have the right layer and text object selected.

With the text layer active press Ctrl+G to put it inside a Group Layer (a folder), which you can see in the Layers list. Now to add an existing image simply drag and drop the image file from your file browser into the canvas and choose Insert as New Layer in the menu that appears. Make sure it's in the group and above the text layer - If not, drag it there from inside the list.

You can limit the visibility of a layer depending on the opacity of the layers below it with Alpha Inheritance. With the new image layer active, click on the little α symbol located on the right side of each layer in the Layers list. Now the image will only be visible inside the text. Making the group is needed so it only uses opacity from the text, not the background. With this method you can freely move, transform and crop it with the respective tools found in the Toolbox and also make a selection with one of the Selection Tools, then press the Delete key to erase the selected part. You can even work non-destructively, if needed, by using a Transparency Mask.

If you want to paint something yourself make a new Paint Layer above the others with the + icon found right below the Layers list, check the α on it and use the Brush Tool. Any brush you choose can also become an eraser by just pressing the E key.

If you want a partially transparent background, edit or delete the lowest layer. The white and grey checkerboard pattern indicates that part of the document being see-through.

Finally, once you're done don't forget to save the document as a Krita file. Then export it as PNG or TIFF image - those are likely preferred over JPEG because they are lossless and can store a transparency channel.

What you want to do is totally doable in krita and is actually quite simple. I'd recommend that you go ahead and play, which is the best way to learn. No matter how much of a mess or how many mistakes you make, you'll learn something useful and you'll (mostly) remember it.

I'd be surprised if a t-shirt could hold/carry an image with better than 100 dpi resolution but Snudl's advice about 300dpi or higher for original work is good because then you can use it for printed posters, etc. After you've finished the image, you can scale down a copy to 100dpi, print it onto A4 paper and hold it at arms length to see if it looks good. You may lose the fine details of stars and nebulae so you may end up having to use a less detailed picture to avoid disappointment. You'll find out.

The use of alpha inheritance would give a simple and elegant final image structure (in terms of number of layers) but it is tricky to understand at first. A simpler technique that requires more intermediate work is to use the text image to make a transparency mask and apply that to your colour/galaxy-image layer.

You'll figure it all out eventually and you can always come here for information and advice. Good Luck and please post a picture of your final design