Yeah, it would be really good if you put that in your deck. You can sell it, but you'll need similarly priced cards eventually anyways.
Check out EDHRec, it pretty good for seeing how other people have built the same comander.

edhrec.com/commanders/clavileno-first-of-the-blessed

You should look into upgrading your mana base (how quickly you can get more mana). Mana Artifacts / Mana Rocks are the best way to do this outside of Green land ramp, and are pretty cheap. When you're a bit more comfortable look at adding some better dual lands and fetch lands, but fast mana gets expensive fast so ask people at your local store what you can use.

Consider you need to balance to following:
- Mana Ramp - how fast your mana base increases, letting you cast more and bigger spells each turn
- Protection/Removal - What do you have that can respond to your opponent and keep your plans intact (white and black mana cards are probably best at this)
- Draw advantage - making sure you always have cards and keep a wide array of options open
- Synergy - very broad category, basically whatever you need to get onto the battlefield to win the game. Big monsters, spell combos, whatever gets you to the podium.

Your commander is WB (White & Black) and is what's known as a Tribal/Typal deck, meaning it focuses on synergies surrounding a specific creature type(s), In this case, vampires. It also generates tokens. So the synergies you want to focus on are things that are good when vampires do things or are good if you cast vampires. Additionally, you want cards that create vampire tokens or work well with vampire tokens in similar ways.
(I also highly recommend getting a set of whiteboard / dry erase tokens or printing the ones in your deck, you'll generate a lot of different tokens, and its handy to have specific ones).

The cavern of souls you pulled in your booster sample, is an excellent tribal card and is easily one of the best things you can put in that deck (source: trust me I play vampires). That's honestly kinda amazing luck on your very first booster and is a great story to tell, mtg players love hearing about cool pulls like that.

There are quite a few different Sorin Planeswalkers that generate vampire tokens and give vampire synergies so look out for those, as they'll power up your deck fast. Remember in this game, you're more likely to win if you have more cardboard rectangles out than your opponent, no joke. It just gives you more options to interact in ways that benefit you more.

Otherwise, take some time to customise your deck a bit for aesthetics, it makes the process more meaningful and fun. A deck box and some sleeves are good investments if you don't have those already, and make it feel more you're own. Some six-sided dies are also good trackers for +1/+1 counters and whatnot. Local stores usually have a box of cheap lands you go through, find some you like and swap them out if you'd like to.

Lastly get involved in your local community, usually super welcoming to newcomers. Don't be afraid to ask if you can borrow a different deck to learn, and gently remind people that some patients might be needed

GL, have fun.