Tree: If you're anywhere near an Ikea, they normally do a tree (natural, fresh lovely smelling!) for £25, but with a £20 voucher (which you can use between Christmas and New Year, when the sales are on). All sorts of sizes and shapes. And, about two weeks before Christmas, they tend to drop their prices to £20, so (with the voucher) you've effectively got a free tree.
IMHO fresh trees are worth the small amount of needle drop (which should be minimal, anyway, if you use a tree stand with a water reservoir) - they smell so good!
In their lots are also a fair amount of pine boughs that have been cut off - I usually take some (the sales staff are grateful that I'm helping them to get rid of "rubbish") and make our door wreath out of it (sturdier wire coat hanger turned into a circle). I also add:
Holly and ivy: which are free, if you wake up before your neighbours do! 
These last longer (than pine boughs) even in a warm house, so I add them to my mantle garland.
Ribbons and things: can come from pound shops and similar, at least initially. A card full of bows (ostensibly for putting on gifts) can look good on a tree. If you're a member of Costco, they have amazingly lush wired ribbon on big spools. A bit pricey but well worth it.
Baubles and ornaments: Make these! They'll be lovely if they're more personal. And DS's crafty things that came home from school take pride of place on my tree. If you want more vintage ornaments (maybe even duplicating what your mum might have had), charity shops are great for a good rummage.
I like to buy things in after Christmas sales, then pack them away when my decorations come down. I'll usually forget about them until I unpack, then they're like a lovely "early present" for me 
Have fun with your very first YOUR Christmas! And don't feel like you have to buy eh-heh-heverything in the first year. But YY to PPs' suggestions of building up your collection year-on-year: you'll end up with a more special collection than a "panic buy".
So excited for you!!!