What percentage grey do you have at present? It should say on the side of the box, or failing that on the manufacturers website, the degree to which the dye covers grey.
If your hair is coloured mid brown, that's probably about a level 6 in the l'oreal colour system. Light brown would be around a level 7 (hairdressers call this dark blond, but most people would recognise it as a light brown).
If you buy a 6.00 or a 7.00 in a l'oreal box dye you'll get a neutral result. The second and third numbers represent the tones (warm or cool) rather than the depth (darkness or lightness) of a colour. The second number is the primary tone, and the third number the secondary tone.
Neutral = 00
Warm tones = 3 (gold), 4 (copper), 6 (red)
Cool tones = 1 (blue), 2 (purple)
Cool tones will make the overall colour appear a little darker so take that into consideration when picking your level. Tbh, I'd avoid tones, especially reds, as they can be difficult to mask if it goes wrong.
You can buy salon colours (eg loreal inoa or diarichesse) plus the developer from online hairdressing retailers, but in my experience the box dyes are near enough the same in terms of quality.
You can buy plastic capes to shield your clothes or just cut up a bin bag. Add Olaplex or another bonding treatment if you're concerned about damage. You can buy sample packs of Olaplex or single measures from ebay. Olaplex number 1 goes in with the colour/developer and Olaplex number 2 is used as a conditioning mask following colour.
At home hair disasters usually occur due to bad bleaching jobs, not box dyes. Box dyes are really hard to mess up. You'll be fine, and save ££££.