OK, i've given this explanation before, but here we go again:
The way this usually works:
you create a transaction, using your wallet of choice (altough i strongly discourage the use of online wallets, even such a wallet will work).
The wallet will use unspent inputs to addresses it manages to create the transaction, and broadcast this transaction to the network.
This broadcasted transaction will go to the mempool of the nodes, where it will stay untill it gets added to a block or untill the node decides to drop the transaction (each node can have his own configuration, and the mempool is cleared when the node is restarted to).
On average, one block gets mined every ~10 minutes, one block has a fixed maximum size of 1 Mb. Since the transaction also has a physical size that depends on the amount of inputs that are used by the transaction and the number of outputs that are generated by the transaction, only a limited amount of transactions will physically fit into a block.
A "minimal" transaction (altough this does not exist) of one input and two outputs might have a size of ~225 bytes. This means that only about 4000 transactions will fit into a block, given that all transactions are really small and the block is completely filled. In reality, most full blocks seem to contain about ~2000 tx's (i didn't create statistics for this, i found this number just by looking at the last couple of blocks).
A miner can chose which unconfirmed transactions he will add to the block he's currently working on, but since he wants to optimize his income, it's wise of him to add the transactions that have the highest fee per byte of transaction data.
At the moment
https://btc.com/stats/unconfirmed-tx shows me that there are about 50Mb of transactions into the mempool of btc.com's node.
So, to go back to your original question: if you want your transaction to be added to a block, it might be a good idear to:
1) calculate your transaction size => (in*180 + out*34 + 10) for a P2PKH ("normal") transaction
2) look up a good fee per byte on
https://bitcoinfees.21.co/ (160 satoshi's per byte a the time of writing)
3) multiply the size by the optimal fee, round up and add this fee to your transaction
This way, you have a 90% chance of getting added to one of the next couple of blocks
EDIT: DannyHamilton pointed out that my information on how to estimate transaction sizes is outdated,using inputs from a compressed address only accounts for ~149 bytes instead of 180.link