How do you use it? Do I buy a developer? If so which?

I know this is an old question, but for anybody looking for an answer- Demi - or semi- permanent hair dyes , like this one, don’t have ammonia , thus need a very low developer, anything above 20 volume is too high, otherwise it will oxidize and the color will look darker, paler than the original one. I use a volume 6 for mine , you could go as high as 10, maybe 20 won’t be so bad, but anything above that is not recommended .

Does the 8A have a blue base or purple? Searched everywhere online but cannot find the answer!

The base for 8A is blue. In case you want to add - Each color is formulated specially for the level, so not all A's in Wella Color Charm are blue based.

How many oz of activating lotion do I use? And do I use a full tube in one coloring? (hair not super long).

I have medium length hair (2-3 inches past shoulders) I used 4 oz of activating lotion and 2 oz (which was entire tube) of the coloring. I had a small amount left over when finished so I think that's a good amount. The directions says 2 parts lotion to 1 part color

Is this hair color tube 2oz and i order activating lotion 4oz separately? mix them and apply how - does it have a squeeze bottle? sorry i am a guy

You purchase each separately. If you’re going to do this regularly, get as big a bottle of activating lotion as you can. Squeeze bottle is separate too. Squeeze bottle should have measuring markings so you can get the proportions correct. Mix color and activating lotion thoroughly in the squeeze bottle by shaking vigorously (with your finger over the top so it doesn’t spray all over.). I’ve been coloring my roots with these products for decades. I often mix dark brown with some golden browns and or a little of a dark reddish.