Try a pencil first much cheaper. . . Lol

What your planning on cutting will determine which cutters and size you'll want to use.
If Aluminium/plastics then 6mm is a good all round size and single or twin flute are best, 3 flutes at the most. Carbide will cost more than HSS but last longer and allow higher feeds and DOC. HSS are good for learning has they are cheaper. I use Cutwel Limited | Cutting Tools & Machine Tool Accessories for my metal/plastics tooling.

If cutting MDF then ONLY use carbide has MDF dulls HSS in no time. Size will depend on what your doing and often wood cutters come in imperial shank sizes so 1/4" 3/8" 1/2" are common shanks also 8mm. Actual cutter sizes can be quit large with wood bits and 1/2" shanks can yield 50-60mm cutter widths.
I use Wealden Tool Company for wood/mdf bits.

Most spindles only come with 1 collet often 6mm so I'd Buy a set of Collets when you buy spindle and you'll be sorted for every shank size upto 13mm, Only need one set for both imperial and metric shanks.