Sorry for my initial posts... they were a bit inaccurate. I missed the part where you were defining which rows you wanted to insert into. I think a large part of your problem is that the numeric you are entering for rental_amount is enclosed in quotes (its not a string, so they need to be removed), the space in the rental_amount still needs to be removed, then you will need to make sure your dates are in the correct format that Oracle expects.
I recreated your table in MySQL and added a line to test...
mysql> desc tbl_lease;
+----------------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+---------------+------+-----+---------+-------+
| LEASE_ID | char(14) | NO | | | |
| RENTAL_AMOUNT | decimal(10,0) | YES | | NULL | |
| BEGIN_DATE | date | YES | | NULL | |
| END_DATE | date | YES | | NULL | |
| LEASE_SIGNDATE | date | YES | | NULL | |
| LEASE_NAME | varchar(15) | YES | | NULL | |
+----------------+---------------+------+-----+---------+-------+
6 rows in set (0.05 sec)
mysql> INSERT INTO tbl_lease VALUES ('Gardern1', 120000, '2008-01-01', '2008-01-10', '2009-01-10','Test');
Query OK, 1 row affected (0.00 sec)
mysql> select * from tbl_lease;
+----------+---------------+------------+------------+----------------+------------+
| LEASE_ID | RENTAL_AMOUNT | BEGIN_DATE | END_DATE | LEASE_SIGNDATE | LEASE_NAME |
+----------+---------------+------------+------------+----------------+------------+
| Gardern1 | 120000 | 2008-01-01 | 2008-01-10 | 2009-01-10 | Test |
+----------+---------------+------------+------------+----------------+------------+
1 row in set (0.00 sec)